Social bike race costume - Morph Suits
I would look for other alternatives.

These suits are very "one general size" type. Meaning they don't fit very well and have quite a bit of chafing as a possibility. I wore one for a fun 10K a few years back and I am glad I wore my trainer undies with them. I got chaffing around the armpits and the visibility was such that I wouldn't want to be on a single track course. If you must pick a theme to do this event, skip the morph suits.

IMHO
Mountain bike racing and the general silliness of a themed event poses several safety challenges. Usually people are less inclined to take their surroundings into account when their having fun. Add that with trail conditions and people on mountain bikes and I think you'd be asking for collisions and crashes.