Hmm, interesting.
So you have Kermit as the Hatbox Ghost and Piggy as the original Beating-Heart Bride (which got replaced with Constance Hatchaway).
Beaker as Horace Fusselbottom, the comics name for the groundskeeper.
Gonzo as Gus, the Prisoner of the Hitchhiking Ghosts.
The Main Muppeteer Five as the Singing Busts.
Would like to know what other Muppets got cast as which Mansion characters, I'd go with Janice for Madam Leotta myself.
Funnily enough, I'm working on something similar but that has morphed into something completely different. Because of my counting collector nature, I've been greatly dissatisfied there's no true definitive informative source that tells us or lists who all 999 Happy Haunts are like the full list of Dr. Jumba Jokiba's genetic experiments. So I set to thinking, I'm gonna create a rival to the Haunted Mansion with 1000 haunters where we know who all 1000 are because I had to know who they were in order to approve/add them to the roster. And if I'm going to do this, what better location to have as our main HQ residence rivaling the Haunted Mansion than the Count von Count's castle. Which gave way to the thought stemming from previous Halloween photo contests with the action figure collections on the Palisades Toys forums that I could have various Muppet characters as different monsters like Ernie as the Headless Horseman, Beaker as the Invisible Man, Dr. Teeth as Jack the Ripper and so on. But I'm not limiting it to recognized movie monsters, I'm bringing in all manner of ghosts and ghouls and mythic creatures and characters that I want to have added. And now, instead of a rival to the Haunted Mansion, I find my realm's developed into more of a rival to Castlevania, and I'm okay with that.
From the Haunted Mansion cast I have:
Bruno the Trashman as Bifrons, who takes over the role of the groundskeeper.
Ezra, the Skeleton; Phinneas, the Traveler; and Gus, the Prisoner; collectively known as the Hitchhiking Ghosts.
Stretching Room Portraits:
Crazy Harry as Alexander Nitrokoff, the man standing on the powder keg barrel.
Mama Fiamma as Abigail Patecleaver, the lady sitting on the tombstone.
Muppet pigs as Big Hoggs, Hoggs, and Skinny Hoggs (though their HM names are Hobbes), the quicksand sinking men.
Judy, an AM from the Rainbow Connection finale in TMM as Daisy de la Cruz, the tightrope walker.
Sinister Eleven/Following Eye Portraits:
Vincent Twice as the aging man.
Lady Agatha as the mysterious woman with the opera glasses.
Miss Piggy as Medusa.
Tammy Whynot as seen in the insert for the song It's A Lovely Eleven Morning and Old MacDonald as the couple imitating the American Gothic portrait.
Dr. Teeth as Jack the Ripper.
Nessa, the Grand Witch, from the SST News Flash Holiday Mix-Up sketch.
Blind Pew replacing Captain Culpepper Cline.
The Phantom Five:
Frank Oz Muppet as Dank, the hunchbacked bass player.
Dave Goelz Muppet as Degrave, the drummer.
Jim Henson Muppet as Dim, the banjoist.
Richard Hunt Muppet as Ditchard, the bagpiper.
Jerry Nelson Muppet as Drerry, the guitarist.
As of right now these are all part of my master monster roster, I'm in the process of doing write-ups for each and then get people to help drawing them as full illustrations.
Hope this helps, great to hear from another fellow HM/Muppet fan here.