Unity用Editorスクリプト。移動回転拡縮を同時にできるハンドルを表示します。Editorフォルダに入れてください。コメントにunitypackageも用意しました。質問等あればこちらまでどうぞ。https://twitter.com/TAK_EMI

TAK_Handle.cs

using UnityEngine;
using UnityEditor;
using System.Collections;

[CustomEditor(typeof(Transform))]
public class TAK_Handle : Editor
{
	static bool viewFlag = true;

	bool ctrlFlag = false;
	Vector3 preScale = Vector3.one;

	[MenuItem("TAK_Handle/view %#w")]
	static void view()
	{
		viewFlag = true;

		return;
	}

	[MenuItem("TAK_Handle/view %#w", true)]
	static bool isView()
	{
		return !viewFlag;
	}

	[MenuItem("TAK_Handle/hide %#q")]
	static void hide()
	{
		viewFlag = false;

		return;
	}

	[MenuItem("TAK_Handle/hide %#q", true)]
	static bool isHide()
	{
		return viewFlag;
	}

	public override void OnInspectorGUI()
	{
		//this.DrawDefaultInspector();

		Transform trans = this.target as Transform;

		trans.localPosition = EditorGUILayout.Vector3Field("Position", trans.localPosition);
		trans.rotation = Quaternion.Euler(EditorGUILayout.Vector3Field("Rotation", trans.eulerAngles));
		trans.localScale = EditorGUILayout.Vector3Field("Scale", trans.localScale);

		EditorUtility.SetDirty(this.target);

		return;
	}

	void OnSceneGUI()
	{
		if (!viewFlag || (Tools.viewTool == ViewTool.Orbit))
			return;

		Transform trans = this.target as Transform;

		Event e = Event.current;
		if (e.type == EventType.mouseDrag)
			Undo.RecordObject(this.target, "Transform Object");


		float defLeng = HandleUtility.GetHandleSize(Vector3.left);

		Vector3 pos, scale;
		pos = trans.position;
		scale = trans.localScale;
		Quaternion rot = trans.rotation;

		rot = trans.rotation = Handles.RotationHandle(rot, pos);
		pos = Handles.FreeMoveHandle(pos, rot, defLeng * 0.4f, pos, Handles.RectangleCap);
		pos = trans.position = Handles.PositionHandle(pos, rot);

		bool ctrl = this.ctrlFlag;
		if (e.type == EventType.keyDown && (e.keyCode == KeyCode.LeftControl || e.keyCode == KeyCode.RightControl))
			ctrl = this.ctrlFlag = true;
		else if (e.type == EventType.keyUp && (e.keyCode == KeyCode.LeftControl || e.keyCode == KeyCode.RightControl))
			ctrl = this.ctrlFlag = false;

		if (ctrl)
		{
			Vector3 preScale = this.preScale;
			if (e.type == EventType.mouseDown && e.button == 0)
				preScale = this.preScale = scale;

			Vector3 tscale = scale;
			tscale = Handles.ScaleHandle(tscale, pos, rot, -defLeng);
			Vector3 diff = tscale - scale;

			if (diff.x != 0.0f)
			{
				float pow = tscale.x / preScale.x;
				trans.localScale = new Vector3(preScale.x, preScale.y * pow, preScale.z * pow);
			}
			else if (diff.y != 0.0f)
			{
				float pow = tscale.y / preScale.y;
				trans.localScale = new Vector3(preScale.x * pow, preScale.y, preScale.z * pow);
			}
			else if (diff.z != 0.0f)
			{
				float pow = tscale.z / preScale.z;
				trans.localScale = new Vector3(preScale.x * pow, preScale.y * pow, preScale.z);
			}

		}
		else
			scale = trans.localScale = Handles.ScaleHandle(scale, pos, rot, -defLeng);

		return;
	}
}

ちょっと、しばらく改修できそうにないのと、長くなってきたので、コメントを整理しました。
unitypackageのダウンロードはこちら。
Ctrlキーを押しながら拡縮ハンドルを操作すると「2軸同時操作」になります。

大体修正しました。
現在はローカル座標系にしか対応させてないです。