DC Animated Universe
In the DC Animated Universe (DCAU) Mark played the Joker in a movie that Harley Quinn and Poison Ivy took over. This is an obvious play on fantasy imitating reality because Mark Hamill was the voice for many charters in the DCAU, most notably the Joker himself. Later Mark is depicted in the comic playing the plat of the Joker.
Other Media
Jay and Silent Bob Strike Back
Mark Hamill also played himself playing an odd ball super-villain known as Cockknocker in the film Jay and Silent Bob Strike Back.