I forgot about Stallone's appearance.Um... Sylvester Stallone was a guest star on the Muppet Show, probably Season 3 or later.
Also, not exactly sure how cartoon2-D animated superstars like Bugs Bunny and/or Daffy Duck could've appeared as guest stars on the Muppet Show in 1980.
As for Bugs and Daffy, they could have easily appeared as guest stars. Roger Rabbit wasn't the first time live action and animation were used together. Disney did it in 1946 on Song of the South, so I see no reason why it couldn't have happened in 1980 or before.