The only function of the caps would be to act as a "spring" between the engine and the motors. Much like the function of a starting capacitor on an AC electric motor.
I doubt the size and weight of the capacitors would need to be anywhere near as much as a bank of lead/acid or lithium batteries...nor the cost.
Plus if properly matched, I'm betting they would last a long time.
Formula race cars use capacitors/flywheels to give a little extra boost.