Here's a Greenspeed in action:
Saw these guys on Mount Diablo a while ago - the older guy is the younger guy's dad. I'm not sure if the drivetrain is locked, requiring Dad to pedal at the same cadence as Son, but Son told me that Dad couldn't ride a regular bike because of physical limitations. Not hard to imagine someone doing RAGBRAI on one of these... probably already been done.
Not a very good project for an engineering class since the solution is already complete - but maybe you could figure out some customization for your subject's particular disabilities.