以前は動作していましたがセキュリティチェックの影響で動作せず。CC0で置いておくので必要に応じて編集して使ってください

jp.lilxyzw.unityversionfaker.asmdef

{
    "name": "jp.lilxyzw.unityversionfaker",
    "rootNamespace": "",
    "references": [
        "VRC.SDKBase.Editor",
        "VRC.SDK3A.Editor"
    ],
    "includePlatforms": [
        "Editor"
    ],
    "excludePlatforms": [],
    "allowUnsafeCode": false,
    "overrideReferences": true,
    "precompiledReferences": [
        "VRCCore-Editor.dll",
        "VRCSDKBase.dll",
        "VRCSDKBase-Editor.dll",
        "0Harmony.dll"
    ],
    "autoReferenced": false,
    "defineConstraints": [],
    "versionDefines": [],
    "noEngineReferences": false
}

UnityVersionFaker.cs

// This code is licensed under CC0 1.0 Universal.
// https://creativecommons.org/publicdomain/zero/1.0/
#if UNITY_EDITOR && UNITY_2022_3
using HarmonyLib;
using System.Collections.Generic;
using System.IO;
using System.Reflection;
using UnityEditor;
using UnityEngine;
using VRC.Core;
using VRC.SDKBase;
using VRC.SDK3A.Editor;
using VRC.SDKBase.Editor.Api;
using System.Reflection.Emit;

namespace jp.lilxyzw.unityversionfaker
{
    [HarmonyPatch]
    class UnityVersionFaker : VRC.SDKBase.Editor.BuildPipeline.IVRCSDKPreprocessAvatarCallback
    {
        static readonly string VERSION_CURRENT = Application.unityVersion;
        const string VERSION_SUPPORTED = "2022.3.22f1";
        const BindingFlags flag = BindingFlags.NonPublic | BindingFlags.Static;
        public int callbackOrder => 999999;

        public bool OnPreprocessAvatar(GameObject obj)
        {
            if(EditorApplication.isPlaying) return true;
            //if(!EditorUtility.DisplayDialog("UnityVersionFaker", "VRChatが推奨する以外のUnityバージョンを使用してビルドしようとしています。いかなる問題が発生しても自己責任であり、VRChatや開発者に問い合わせず自分で解決する必要があります。ビルドを続行しますか？（このダイアログは毎回表示されます。非表示にするにはプログラム自体を編集してください）", "Yes", "No")) return false;

            obj.GetComponent<VRC_AvatarDescriptor>().unityVersion = VERSION_SUPPORTED;
            typeof(VRC.Tools).GetField("_unityVersion", flag).SetValue(null, UnityVersion.Parse(VERSION_SUPPORTED));
            (typeof(VRCApi).GetField("Headers", flag).GetValue(null) as Dictionary<string, string>)["X-Unity-Version"] = VERSION_SUPPORTED;
            if(typeof(VRCSdkControlPanelAvatarBuilder).GetField("_instance", flag).GetValue(null) is VRCSdkControlPanelAvatarBuilder builder)
            {
                builder.OnSdkBuildSuccess -= RewriteBundleHeader;
                builder.OnSdkBuildSuccess += RewriteBundleHeader;
            }
            return true;
        }

        void RewriteBundleHeader(object sender, string path)
        {
            int i = 0;
            var bytes = new byte[64];

            // Copy FileInfo
            var fi = new FileInfo(path);
            var timeC = fi.CreationTime;
            var timeW = fi.LastWriteTime;

            using var fs = new FileStream(path, FileMode.Open);
            fs.Read(bytes, 0, 64);
            fs.Seek(0, SeekOrigin.Begin);

            // Header - "UnityFS     5.x.x "
            for(; i<18; i++) fs.WriteByte(bytes[i]);

            // Header - Unity Version
            i += System.Text.Encoding.ASCII.GetBytes(VERSION_CURRENT).Length;
            fs.Write(System.Text.Encoding.ASCII.GetBytes(VERSION_SUPPORTED));

            // Header - Other
            var a = 64 + Mathf.Min(i - (int)fs.Position, 0);
            for(; i<a; i++) fs.WriteByte(bytes[i]);
            fs.Close();

            // Revert FileInfo
            new FileInfo(path){CreationTime = timeC, LastWriteTime = timeW};
        }

        [InitializeOnLoadMethod]
        static void DoPatching()
        {
            typeof(VRC.Tools).GetField("_unityVersion", flag).SetValue(null, UnityVersion.Parse(VERSION_SUPPORTED));
            (typeof(VRCApi).GetField("Headers", flag).GetValue(null) as Dictionary<string, string>)["X-Unity-Version"] = VERSION_SUPPORTED;
            new Harmony("jp.lilxyzw.unityversionfaker").PatchAll();
        }

        [HarmonyPatch(typeof(VRCApi), "UploadFile"), HarmonyPrefix]
        static void PrefixUploadFile(ref string friendlyFileName) => friendlyFileName = friendlyFileName.Replace(VERSION_CURRENT, VERSION_SUPPORTED);

        [HarmonyPatch(typeof(Application), nameof(Application.unityVersion), MethodType.Getter), HarmonyTranspiler]
        static IEnumerable<CodeInstruction> PatchUnityVersion(IEnumerable<CodeInstruction> instructions)
        {
            yield return new(OpCodes.Ldstr, VERSION_SUPPORTED);
            yield return new(OpCodes.Ret);
        }
    }
}
#endif