They have done something similar to that I believe 2 times. I think there was an item in the brawler game mode (forgot what the mode was called, or the item actually) but I could of sworn there was an AP item that can change what you look like to the enemy I think...maybe I'm remembering that one wrong. Though I know that they have recently did something like you asked with the new doom bots, where the enemy doom bots would look like small devil teemos and remain like that until you hit them or they hit you (even changed their names to).
Making a champion that works like that would be pretty cool though some could say that
can some what do that by making a clone anywhere she wants.