How to provide undo for EditorScript changes to a prefab?

I'm writing an Editor Script (or ScriptableWizard) to modify a prefab -- specifically add child GameObjects to the prefab. I wish to make this operation undoable but cannot determine how.

I've tried Undo.RegisterUndo(), Undo.RegisterCreatedObjectUndo() and Undo.SetSnapshotTarget() + Undo.CreateSnapshot() + Undo.RegisterSnapshot().

Here's the code:

     [MenuItem("Tools/Add child to prefab", true)]
     static bool validateAddChildToPrefab() {
         GameObject go = Selection.activeObject as GameObject;
         if (null == go) return false;
         
         return PrefabUtility.GetPrefabType(go) == PrefabType.Prefab || PrefabUtility.GetPrefabType(go) == PrefabType.ModelPrefab;
     }
 
     [MenuItem("Tools/Add child to prefab")]
     static void addChildToPrefab() {
         GameObject go = (GameObject) Selection.activeObject;
         
         // register undo -- my question: how to get this to work?
 //        Undo.RegisterUndo(go, "Add child to prefab"); // does nothing
 //        makeUndoFor(go, "Add child to prefab"); // (see below) does nothing
 //        Undo.RegisterCreatedObjectUndo(go, "Add child to prefab"); // error: Undo Created Object may not be performed on persistent or non-default hide flag objects
 //        Undo.RegisterSceneUndo("Add child to prefab"); // does nothing (not in scene so wasn't hopeful)
 
         GameObject clone = (GameObject) PrefabUtility.InstantiatePrefab(go);
         GameObject child = new GameObject("My child");
         child.transform.parent = clone.transform;
         PrefabUtility.ReplacePrefab(clone, go, ReplacePrefabOptions.ReplaceNameBased);
         
         // Remove the scene instance of the prefab
         DestroyImmediate(clone);
     }
 
     private static void makeUndoFor(Object root, string actionName) {
         Object[] objectsToRecord = EditorUtility.CollectDeepHierarchy(new Object[]{ root });
         Debug.Log("Creating UNDO point for "+ objectsToRecord.Length +" objects");
         Undo.SetSnapshotTarget(objectsToRecord, actionName);
         Undo.CreateSnapshot();
         Undo.RegisterSnapshot();
     }

p.s. If there's a better way to add child GameObjects to prefabs (i.e. without instantiating), I'd really like to know! Thanks