Winston,
If you are planning on doing this at night I would definitely avoid the rifle. I used to race the NORBA circuit including some of the 24 hour races (Canaan, WV, Moab) so I did a lot of riding at night for training. Riding at night isn't at all like riding in daylight.(duh!) During the day you can usually tone down your riding so as to avoid falls if you need to. At night you some times crash seemingly for no reason at all. Even with the best lighting systems like Nightrider or Turbocat twin-light systems AND a helmet mounted light as well you just don't see everything you do during the day. Shadows do funny things to your perception of the trails and you have almost zero periphery.
A fall would certainly do bad things to your Guide Gun. If this is for protection from cougars, don't bother. If you were being stalked by one of them at night, you'd never know it. They would probably only attack you if they are EXTREMELY hungry and if you are alone. You would never have a chance of getting your GG into action.
Carrying the revolver in a fanny-pack style holster makes a lot of sense for human predators. I've known guys who've been robbed of their expensive bikes while out on training rides. There was a well publicized mugging a few years ago where a group of teens robbed a woman in her twenties while she was out for a ride on her new mountain bike. They beat her pretty bad and stole her bike which was worth about $1200. She was in a coma for several days and the muggers were never caught. This happened during the day. Do not ride alone at night!
Just my two pennies worth, Patrick