Rick Miller has this one-man show in which he performs, as his site claims, 50 unique Simpsons characters in an interpretation of Macbeth. What apparently started off as a back-stage joke became a full-length show, and I think it became a pain in Fox's neck for a while because of the copyright issues.
Miller not only gets away with using the characters' voices - and likenesses, as he has pictures of the characters pop up in the background during the show - but he has met Matt Groening personally and has his blessing. There are links on the MacHomer site to educational modules to help teach and adapt Shakespeare for children and high school students. He's been doing all of this for a while, but he hasn't come to my neck of the woods on his tours in a long time. I think I'm going to buy a DVD of it.