Yeah, Disney is pretty picky about this. You might get away with it if the costumes aren't too over the top, but I wouldn't risk the disapointment as they are not offically allowed.
It's not because they don't want people having fun, but they are afraid other guests could confuse you with CM's, etc. Some people might take advantage of that fact.
What do the costumes consist of? Is there make-up, etc? If you really had your heart set on it, like really really, I'd tell you to go to Guest Services or directly to ToT and speak to one of the managers there and ask if you could get in costume just long enough to ride. There are bathrooms right near ToT and you could get into costume just for the ride...you'd be asking a big favor, but you never know.
NED
