Oh that wouldn't cause drag if it was designed to work as a brake. What the issue is, is that to get 10% of the energy you use per "tank" of electricity you would need to stop every 3 city blocks. This all vary's from the speed your braking at, and weight.
As for the quick charge system, I'm not sure about it. It tends to kill batteries when you shove more amps into them faster than what it should normally handle. Works more for power tools because they aren't used regularly like a commuting bike would. I could be wrong though.
If you ask me, the easiest way to get 300mpg is to be aerodynamic. But that's not really the goal of the thread so maybe a combination of home charging and the gas engine is the best.