There's an MIT spinoff called Top Flight that was working on a hybrid quadcopter, IIRC. I think the main reason you don't see them in the hobbyist space is size floor (you need a pretty big machine to make the weight overhead of the motor, generator, and charge circuitry work out) and money.

Hell, you could have a small buffer ultra capacitor or battery providing the motors with electricity while a gas engine at constant RPM recharges it. The whole system would have to have a very high conversion efficiency though...