I'm not going to lie, it did not go well. Leo was sad/angry/frustrated at the thought of parting with his things and insisted that he did not want to donate anything.
After a LOT of talking and reassurance he did select a few items that "someone else might really enjoy" - but I was surprised at how difficult it was for him. He's a pretty generous kid - but this somehow struck a nerve with him. Perhaps it's a skill that needs to be practiced more frequently.