I'm no electronics genius.....but I don't see it working out too well.
The people that use these heavy bikes probably aren't giong to be putting out much wattage to begin with and since nothing is 100 percent, there will be loss.
The ultra capacitor will be drained to charge a battery, and there will be more loss.
The battery will power the motor, drive --more loss. Will the net gain be worth the cost of a very special bike, ultra capacitor, and all of the other associated hardware?
I'm not seeing it.