Last year I led one of the groups participating in a club century. One guy in my group was missing most of his right forearm and had no right hand. He had some sort of system that allowed him to operate both brakes with one hand. He also had a 2x drivetrain operated with one hand, but I think there were dual controls for that. Wish I could give specifics, but I didn't question the guy about the mechanics of his setup. Just wanted to note that there are solutions out there.