OK, lemme clear up a few things.
1 Last I heard, Dreams & Visions had the license for Dark Crystal merchandise, don't think anything's come of it so far.
2 Seems Palisades got the license for Labyrinth, since they'll be making a 12-inch figure of David Bowie as Jarreth the Goblin King for next spring 2006.
3 There was no independent fan-based company that "was" to make figures of Sal Manilla based on the prototype.
What happened was... Ken Lilly posted the lineup of what would've been from S10 to S14 a few months back. Good thing we got it posted in a thread here, since the boards there got wiped squeaky clean again.
WiGgY, perhaps you know of him? Well, he spearheaded a group of forum members to get together and do this.
They'd make custom sculpts of all the would've been S10 to S14 figures that didn't get released... Send those off to a master caster like Junkers from the Palisades forums. He'd then make the steel cast and mold the figure. After, it'd go to someone else for painting and inspected by the various people involved for reviews. The figure would then be mass reproduced by the same fans for purchase by the rest of the fan community. Unfortunately, there were some who poo-pooed the project, though if I had won the Sal prototype auction it'd have gone to them for their endeavors. And the whole thing's been pretty much abandoned now.
4 As far as other prototypes of Muppet figures, there are the ones on auction at eBay. The ones that were never refined for final action figure form include the trio from Vet's Hospital, Sal Manilla, Gobo Fraggle, and Baby Kermit and Baby Piggy from TMTM.
Hope this helps.