More power to you if you want to play, but here are a couple of thoughts for you.
You mention that when you brake a car, the brakes get hot. With dynamic braking, the alternator is used to slow the wheels instead of the brakes. The energy of the rolling car is converted by the alternator into battery charging current. Does this sound like alternators don't require any power to turn them.
Have you ever jump started a car? If you have the engine idling on the donor car when you hook up the battery cable, you can hear the donor car's engine "grunt" when you hook up the dead battery. The alternator is applying this load to the engine as it tries to charge the dead battery. Does this sound like alternators don't require any power to turn them.
Motorcycle racers remove the magneto/alternator from thier bikes and switch to a "total loss" battery powered igniyion system, just to eliminate the drag of the alternator and get a little more speed.
I am sure you will be able to rationalize this and go ahead with your project so good luck to you.