There have been two instances where they brought back celebrity guest stars: Lance Armstrong (seasons 12 and 13) and Joan Rivers (seasons 12 and 15).
For "Elwood City Turns 100," Muffy invited past guest stars. Larry King was the only guest star to not be animated.
They have even had "celebrity guest stars" that I've never even heard of. The Finnish ABBA-like band that played Binky, Arthur Ganson, Koko Taylor, Taj Mahal, Frank Gehry, Mike Finke (I did see him on the CBS Evening News recently).