I coach Special Olympics cycling. While the folks on my team all ride bikes/trikes without any significant modifications, I always marvel at some of the rides I see at the State competition. Lots of very cool stuff being done to make bikes suitable for the athletes.
I know an oldtimer who used to love riding, but his left arm has been injured and has since atrophied to such a degree that he can't do a whole lot with it. The right arm is still plenty strong. He's interesting in riding again; I think i'm gonna give him my old mtb, but with one of those one-lever-two-cables brake lever, and some old stem shifters mounted as thumbies on the bar, so he can control both derailers with his right hand. It's an idea I gleaned from some of the Olympics guys, but they tend to do a trigger/gripshifter combo on one side....