Custom Editor: How to get a reference to an existing object in the scene at the mouse position after drag-and-dropping a prefab into the scene?

This is what I'm trying to accomplish: On drag-and-dropping a prefab into the scene, if another object with the same tag exists at that position (this is a 2D scene, mind you), delete it and replace it with the new object (keeping the old object's position in the scene and the hierarchy).

The part I'm having a problem with is checking if any existing objects are at the drop position and getting a reference to them, and after a couple hours of googling and going through the manual I'm still stumped.

     [CustomEditor(typeof(CommonTile))]
     public class CommonTileInspector : Editor
     {
         void OnSceneGUI()
         {
             if (Event.current.type == EventType.DragExited)
             {
                 GameObject oldObj = this.getObjectAtDropPosition();
                 GameObject newObj = ((CommonTile)this.target).gameObject;
 
                 if (oldObj != null && oldObj.CompareTag(newObj.tag))
                 {
                     newObj.transform.SetParent(oldObj.transform.parent);
                     newObj.transform.position = oldObj.transform.position;
                     Undo.DestroyObjectImmediate(oldObj);
                 }
                 Event.current.Use();
             }
         }
 
         private GameObject getObjectAtDropPosition()
         {
             // this is the part that stumps me
         }
     }

HandleUtility's PickGameObject and PickRectObjects functions look promising, but I can't figure out how to get a mouse position at this point; getting Event.current.mousePosition just gives me the same negative numbers regardless of where I drop the object (likely related to the object not actually existing in the scene until after it's been dropped).