I'm a little skeptical about any significant savings, here. The alternator's load on the engine is directly proportional to its current output. That means that when it's just producing enough electricity to run the onboard electronics (once the battery is topped off) it's using far less than a horsepower, maybe as little as 250 watts, about a third of a horse. I can see some potential mileage improvements by going with the original idea of using it as a regenerative engine brake, but I really can't see how it could help under any other scenario.
