One of my brainstorms is the 2-story backpacking tent, and I've had problems working the details out there as well.

On mounting the bike above...put a rubber tire around the upper sprocket, maybe 16" tire or so. Fix the seats back a little further. Tip it over frontwards with the guys standing, and get started like that. Then on 3, they do a wheelie and flip it to the back wheel and off they go.