Heck, Dan Castellaneta's done it (with not-so-sexy results IMO)....
Why not have Harry Shearer and Christopher Guest write an episode? Guest is a very funny man, and Shearer's comedic talent for writing is underappreciated. "Waiting for Guffman" and "Best In Show" IMO show how to write a real tongue-in-cheek satire where the humor comes more from the situation than from the dialogue (at least directly). Guest's type of humor _never_ makes the characters explain what the joke is, or what's funny about a situation. His characters are very interesting, and they're just... THERE. They are inherently funny, because they're satirical; not because they fart, or say the word "fart" or "testicles", or because they crack any type of joke, or whatever.
These two were original members of Spinal Tap, and the reputation and humor of that movie are becoming legendary. Of course, we know how connected Shearer is to The Simpsons (if you don't, drop and gimme 20; then take 5 laps around the pool).
Just an idea.