How to make drag-drop detecting Editor script?

Hi, I'm wanting to detect when an object or objectS are drag/dropped onto another object to reparent them. I'd like to pop up a UI to select 'keep local transforms?' so that the objects will reposition themselves relative to the object onto which they are dropped, rather than recompute position to retain their world positions. Do I use a CustomEditor? Would that be for type Transform? Do I use OnSceneGUI? I ask cuz what I have is not being called:

 using UnityEngine;
 using System.Collections;
 using UnityEditor;
 
 [CustomEditor(typeof(Transform))]
 public class Reparent : Editor {
 
     private GameObject draggedObj;
     public void OnSceneGUI()
     {
         if (Event.current.type == EventType.DragUpdated || Event.current.type == EventType.DragPerform)  // gets here....
         {
             DragAndDrop.visualMode = DragAndDropVisualMode.Copy; // show a drag-add icon on the mouse cursor EXCEPT IT NEVER GETS HERE