You asked, so here's why this won't work:
"fit a pedal-powered generator". Where do those pedals fit? Basically down and in front. Take a look at some recumbent bicycle designs - preferably the more upright ones. Take a look at some wheelchair designs - standard legs out in front hospital-style, and some more active ones like tilite.com. Your pedal chain rings and crank-arms are going to have to fit somewhere very clever indeed to avoid them or the legs sticking out in front in a very awkward manner. Even standard hospital-style wheelchairs are a PITA to maneuver about due to the legs banging into everything, by adding the ability to pedal, you've just made that PITA far, far worse.
Leg strength - You're asking the compromised legs of someone who needs a wheelchair to produce power equivalent to an able-bodied person. That's not going to happen for a long, long time while atrophied muscles get stronger. You device needs to be enjoyable enough during that time, or human natures says, it will be abandoned.
Generating power - Most electric bicycles are just power assist for climbing hills. I don't think full-power generation is currently practical.
Power wheelchairs are hardly restricted to slow short-range trundling. They're 24-volt, and can reach speeds of at least 25km/h have ranges than can be extended by greater battery capacity, up to at least 30km. (Speed and range maximums may be higher - numbers are just from some individuals I know).