Just like most effects in the Mansion it's nowhere as complicated as that. It's simply two stationary lights/projectors positioned above and behind the Doombuggies in that stretch of track, pointing forward into the ride path of the Buggies. As the buggies roll along, they pass through the field of the projection, and the hand shines on the back of each Buggy as it goes by. (Think of your car driving past a streetlight)
A more elaborate system could've used an array of 3 or 4 video projectors that could project images that would follow each Buggy and give an animated hand pointing the way.
As for the hand vs. skeleton, perhaps they could take a cue from the changing portrait in the foyer, and have the first hand be normal, the second hand be decaying, and the 3rd hand (if there is one) be a skeleton...
-Rob