A radial fill element that can be used in Unity UI Toolkit, makes use of MeshWriteData to create a mesh which will only be visible within the confines of its parent due to the overflow attribute.

RadialFillElement.cs

using UnityEngine;
using System.Collections;
using System.Collections.Generic;
using UnityEngine.UIElements;

namespace NS.RomanLib
{
    public class RadialFillElement: VisualElement, INotifyValueChanged<float>
    {
        protected float m_value = float.NaN;
        public void SetValueWithoutNotify(float newValue)
        {
            m_value = newValue;
            radialFill.MarkDirtyRepaint();
        }

        public float value 
        {
            get
            {
                m_value = Mathf.Clamp(m_value, 0, 1);
                return m_value;
            }
            set
            {
                if (EqualityComparer<float>.Default.Equals(this.m_value, value))
                    return;
                if (this.panel != null)
                {
                    using (ChangeEvent<float> pooled = ChangeEvent<float>.GetPooled(this.m_value, value))
                    {
                        pooled.target = (IEventHandler) this;
                        this.SetValueWithoutNotify(value);
                        this.SendEvent((EventBase)pooled);
                    }
                }
                else
                {
                    this.SetValueWithoutNotify(value);
                }
            }
        }
        public float width { get; set; }
        public float height { get; set; }
        public Color fillColor { get; set; }   
        public float angleOffset { get; set; }
        public string overlayImagePath { get; set; }

        public enum FillDirection
        {
            Clockwise,
            AntiClockwise
        }

        public FillDirection fillDirection { get; set; }
        private float m_overlayImageScale;
        public float overlayImageScale
        {
            get
            {
                m_overlayImageScale = Mathf.Clamp(m_overlayImageScale, 0, 1);
                return m_overlayImageScale;
            }
            set => m_overlayImageScale = value;
        }
        
        private float radius => (width > height) ? width / 2 : height / 2;

        public VisualElement radialFill;
        public VisualElement overlayImage;

        public new class UxmlFactory : UxmlFactory<RadialFillElement, UxmlTraits> { }

        public new class UxmlTraits : VisualElement.UxmlTraits
        {
            UxmlFloatAttributeDescription m_value = new UxmlFloatAttributeDescription() { name = "value", defaultValue = 1f};
            UxmlFloatAttributeDescription m_width = new UxmlFloatAttributeDescription() { name = "width", defaultValue = 20f};
            UxmlFloatAttributeDescription m_height = new UxmlFloatAttributeDescription() { name = "height", defaultValue = 20f};
            UxmlFloatAttributeDescription m_angleOffset = new UxmlFloatAttributeDescription() { name = "angle-offset", defaultValue = 0};
            UxmlColorAttributeDescription m_fillColor = new UxmlColorAttributeDescription() { name = "fill-color", defaultValue = Color.white};
            UxmlStringAttributeDescription m_overlayImagePath = new UxmlStringAttributeDescription() { name = "overlay-image-path", defaultValue = ""};
            UxmlFloatAttributeDescription m_overlayImageScale = new UxmlFloatAttributeDescription() { name = "overlay-image-scale", defaultValue = 1f};
            UxmlEnumAttributeDescription<FillDirection> m_fillDirection = new UxmlEnumAttributeDescription<FillDirection>() { name = "fill-direction", defaultValue = 0 };

            public override IEnumerable<UxmlChildElementDescription> uxmlChildElementsDescription
            {
                get { yield break; }

            }
            public override void Init(VisualElement ve, IUxmlAttributes bag, CreationContext cc)
            {
                base.Init(ve, bag, cc);
                var ate = ve as RadialFillElement;

                // Assigning uxml attributes to c# properties
                ate.value = m_value.GetValueFromBag(bag, cc);
                ate.width = m_width.GetValueFromBag(bag, cc);
                ate.height = m_height.GetValueFromBag(bag, cc);
                ate.fillColor = m_fillColor.GetValueFromBag(bag, cc);
                ate.overlayImagePath = m_overlayImagePath.GetValueFromBag(bag, cc);
                ate.overlayImageScale = m_overlayImageScale.GetValueFromBag(bag, cc);
                ate.angleOffset = m_angleOffset.GetValueFromBag(bag, cc);
                ate.fillDirection = m_fillDirection.GetValueFromBag(bag, cc);

                // Creating the hierarchy for the radial fill element
                ate.name = "radial-fill-element";
                ate.Clear();
                VisualElement radialBoundary = new VisualElement() { name = "radial-boundary" };
                radialBoundary.Add(ate.radialFill);
                ate.radialFill.Add(ate.overlayImage);

                ate.radialFill.style.flexGrow = 1;
                ate.overlayImage.style.flexGrow = 1;
                ate.overlayImage.style.scale = new Scale(new Vector2(ate.overlayImageScale, ate.overlayImageScale));
                ate.style.height = ate.height;
                ate.style.width = ate.width;
                radialBoundary.style.height = ate.height;
                radialBoundary.style.width = ate.width;
                radialBoundary.style.overflow = Overflow.Hidden;
                ate.overlayImage.style.backgroundImage = null;
#if UNITY_EDITOR
                Texture2D tex = UnityEditor.AssetDatabase.LoadAssetAtPath<Texture2D>(ate.overlayImagePath);
                if (tex != null)
                {
                    ate.overlayImage.style.backgroundImage = tex;
                }
#endif
                // Angle Offset determines the rotation of the radialFill VE, overlayImage will use the inverse of this
                // rotation so the image remains upright
                ate.radialFill.transform.rotation = Quaternion.Euler(0, 0, ate.angleOffset);
                ate.overlayImage.transform.rotation = Quaternion.Euler(0, 0, -ate.angleOffset);
                ate.Add(radialBoundary);
            }
        }

        public RadialFillElement() : base()
        {
            radialFill = new VisualElement() { name = "radial-fill" };
            overlayImage = new VisualElement() { name = "overlay-image" };
            radialFill.generateVisualContent += OnGenerateVisualContent;
        }

        public void AngleUpdate(ChangeEvent<float> evt)
        {
            radialFill?.MarkDirtyRepaint();
        }

        public void OnGenerateVisualContent(MeshGenerationContext mgc)
        {
            // default draw 1 triangle
            int triCount = 3;
            int indiceCount = 3;
            m_value = Mathf.Clamp(m_value, 0, 360);
            if (m_value * 360 < 240)
            {
                // Draw only 2 triangles
                if (value * 360 > 120)
                {
                    triCount = 4;
                    indiceCount = 6;
                }
            }
            // Draw 3 triangles
            else
            {
                triCount = 4;
                indiceCount = 9;
                if (m_value < 1)
                {
                    triCount = 5;
                    indiceCount = 9;
                }
            }
            
            // Create our MeshWriteData object, allocate the least amount of vertices and triangle indices required
            MeshWriteData mwd = mgc.Allocate(triCount, indiceCount);
            Vector3 origin = new Vector3((float)width/2, (float)height/2, 0);

            float diameter = 4 * radius;
            float degrees = ((m_value * 360) - 90)/Mathf.Rad2Deg;
            
            //First two vertex are mandatory for 1 triangle
            mwd.SetNextVertex(new Vertex(){position = origin + new Vector3(0 * diameter, 0 * diameter, Vertex.nearZ), tint = fillColor});
            mwd.SetNextVertex(new Vertex(){position = origin + new Vector3(0 * diameter,  -1 * diameter, Vertex.nearZ), tint = fillColor});

            float direction = 1;
            if (fillDirection == FillDirection.AntiClockwise)
            {
                direction = -1;
            }

            mwd.SetNextIndex(0);
            mwd.SetNextIndex((fillDirection == FillDirection.AntiClockwise) ? (ushort)2 : (ushort)1);
            if (m_value * 360 <= 120)
            {
                mwd.SetNextVertex(new Vertex(){position = origin + new Vector3(Mathf.Cos(degrees) * diameter * direction, Mathf.Sin(degrees) * diameter, Vertex.nearZ), tint = fillColor});
                mwd.SetNextIndex((fillDirection == FillDirection.AntiClockwise) ? (ushort)1 : (ushort)2);
            }

            if (m_value * 360 > 120 && m_value * 360 <= 240)
            {
                mwd.SetNextVertex(new Vertex(){position = origin + new Vector3(Mathf.Cos(30/Mathf.Rad2Deg) * diameter * direction, Mathf.Sin(30/Mathf.Rad2Deg) * diameter, Vertex.nearZ), tint = fillColor});
                mwd.SetNextIndex((fillDirection == FillDirection.AntiClockwise) ? (ushort)1 : (ushort)2);
                mwd.SetNextVertex(new Vertex(){position = origin + new Vector3(Mathf.Cos(degrees) * diameter * direction, Mathf.Sin(degrees) * diameter, Vertex.nearZ), tint = fillColor});
                mwd.SetNextIndex(0);
                mwd.SetNextIndex((fillDirection == FillDirection.AntiClockwise) ? (ushort)3 : (ushort)2);
                mwd.SetNextIndex((fillDirection == FillDirection.AntiClockwise) ? (ushort)2 : (ushort)3);
            }

            if (m_value * 360 > 240)
            {
                mwd.SetNextVertex(new Vertex(){position = origin + new Vector3(Mathf.Cos(30/Mathf.Rad2Deg) * diameter * direction, Mathf.Sin(30/Mathf.Rad2Deg) * diameter, Vertex.nearZ), tint = fillColor});
                mwd.SetNextIndex((fillDirection == FillDirection.AntiClockwise) ? (ushort)1 : (ushort)2);
                mwd.SetNextVertex(new Vertex(){position = origin + new Vector3(Mathf.Cos(150/Mathf.Rad2Deg) * diameter * direction, Mathf.Sin(150/Mathf.Rad2Deg) * diameter, Vertex.nearZ), tint = fillColor});
                mwd.SetNextIndex(0);
                mwd.SetNextIndex((fillDirection == FillDirection.AntiClockwise) ? (ushort)3 : (ushort)2);
                mwd.SetNextIndex((fillDirection == FillDirection.AntiClockwise) ? (ushort)2 : (ushort)3);

                if (m_value * 360 >= 360)
                {
                    mwd.SetNextIndex(0);
                    mwd.SetNextIndex((fillDirection == FillDirection.AntiClockwise) ? (ushort)1 : (ushort)3);
                    mwd.SetNextIndex((fillDirection == FillDirection.AntiClockwise) ? (ushort)3 : (ushort)1);
                }
                else
                {
                    mwd.SetNextVertex(new Vertex() { position = origin + new Vector3(Mathf.Cos(degrees) * diameter * direction, Mathf.Sin(degrees) * diameter, Vertex.nearZ), tint = fillColor });
                    mwd.SetNextIndex(0);
                    mwd.SetNextIndex((fillDirection == FillDirection.AntiClockwise) ? (ushort)4 : (ushort)3);
                    mwd.SetNextIndex((fillDirection == FillDirection.AntiClockwise) ? (ushort)3 : (ushort)4);
                }
            }
        }
    }
}