That's pretty neat. Seems like there's a good possibility to make these levels available through GameShark or similar methods (which would then also allow access of the levels via emulator as well), will have to try and see one of these days. Who knows, maybe with that method there are more levels past the nonworking Fun 33 that can be accessed?
Wasn't it you who extracted the Genesis levels into .lvl format from the ROM? How comes you missed them?
Anyway, I had a short glance into the array where the level stats and name are stored, and there are 202 entries in total. Considering there's 180 regular levels and 20 Two-Player levels, I guess there's no space for more hidden levels at least in that index. Doesn't mean there couldn't some unused level layout lying around in the ROM though.
EDIT: btw @finlay: I think 'private room available' isn't that bad, you've got 5 possible pixels to dig at to get into a room, and 3 for getting out. So while it might take a few tries, it isn't all that hard to pull off. When I first saw the level, I didn't find the solution to it obvious either, took me a bit to find it.