My Zero DS will do about 70 miles with its 10kWh of useable juice. That's about 143 wH per mile. I think most people would be lucky to produce 143wH during an hour of peddling. They may get more indicated, say closer to 200wH, but by the time it is rectified to DC and stored in the battery, I think it's a reasonable estimate.
So by my reckoning, a human can charge a Zero at a rate of 1mph.
Nice idea, but unless you have lots of kids with too much energy, it's a non starter.