I have a friend (about 100 pounds) that I used to go pick up with my coaster-brake bike. 63 gear inches, old Schwinn road frame, 1.5" mountain bike slicks...on a standard Blackburn rack. It did make it a little hard for the both of us, her not having anywhere to rest her feet- it made balance sketchy. But so did the fact that we were both cracking up the whole time.

I love your idea, alot. It's inspiring. Good luck.