The losses involved in putting power into a generator by peddling, and then moving that power to a battery, and then taking the poser from the battery to the motor make the work involved MORE than if you were peddling. Think about it. There are actually four energy changes taking place, and NONE of them are 100% efficient. The first involves converting mechanical energy to electrical with the generator. The second is converting electrical energy to Chemical in the battery when charging it. The third is converting chemical energy back to electrical when you want a boost, and the fourth is when you convert electrical energy back to mechancal with the motor.
A generator is (optimistically) about 75-80 percent efficient. Charging a battery and then retrieving the electricity is about 80 percent efficient. The electric motor is about 80 percent efficient. So, in order to provide 500 watt hours to the bike motor, you would have to peddle 500WH / (.75*.8*.8) or, approximately 1042 watts-hours. It's a LOT more efficient to just peddle the bike!
Now, as far as regenerative braking. That idea's been around for a long time. I read an article in the mid-70's in "The Mother Earth News," discussing building a hybrid car, using regenerative braking to help boost efficiency. And, it does help in extending the range of an electric hybrid. (an electric bike is a form of a hybrid. But, for that matter, so is a gasoline powered bike...)