I have a low quality copy of this special on CD at home. It's still very watchable though. And I do remember seeing "The Muppets at Walt Disney World" in 1990 as a child. Raven-Symone (then of "The Cosby Show" fame, later of "That's So Raven" fame) appeared on the special, as you said.
The way it started was that Kermit wanted to take the whole Muppet gang with him for his family reunion at the swamp. They did visit the swamp at first and sang a song with the frogs. When the Muppets find out that Kermit's swamp is right next to Walt Disney World, they all vote to go there and Kermit and Robin go after them. Kermit winds up chasing the rest of the gang all over Disney World, trying to convince them to come back for the rest of the Frog Festival. We get to see a lot of the famed park- with Gonzo and Camilla going off on their own through the laundromat somehow and the Electric Mayhem going off by themselves and winding up playing a song ("Rocking All Around the World") and Fozzie and his mom are there and Beauregard and Miss Piggy are off by themselves- they ride Splash Mountain together, I believe. Rowlf gets put in the dog pound and sings a song there. Statler and Waldorf show up riding on a carriage and sing a song. (My memory may off on some places, but I know all those characters were there)
Anyway, Kermit at some point gets all depressed because he can't find his friends, and I forgot to say earlier that they're all being chased by Charles Grodin, who portrays the security guard (yeah, the Muppets didn't exactly pay to get in- Animal sort of knocked over the gate and they went in!)
He's wandering by himself and feeling really sad that now he can't even find Robin who had gone off to do something. And Raven-Symone walks up and asks why he's feeling so sad and he tells her about his situation and (well, you can hear the dialogue on the MP3) The only other thing Raven says is: after Kermit's left "Too bad he left (or something like that) I was going to tell him it's not easy being green."
And Kermit eventually finds his friends, and they get to meet up with Mickey Mouse who tells the security guard that Kermit and the Muppets are friends of his and to let them go. The Muppets sing one last song together and they all go back for the rest of the Frog Festival (though Piggy's left stuck in the mud at the Chinese Theater, where the Hollywood walk of stars is. (She's trying to sign her name by a star.)
It's a great special and seems like a natural to get released on VHS and DVD now that Disney actually owns the Muppets! Maybe someday. In the meantime, if you'd like a copy of it, send me a PM and maybe we can work something out.