Just to entertain the idea...

A second motor could be attached to the drive train and wired back to the battery. This is as close to a generator as you will find. A brushless motor would be the best bet because of the higher efficiency. It would work similar to a car alternator.

A good experiment would be to attach 2 motors together by their spindles then attach a multi tester to the terminals of on one side and a battery to the terminals of the other motor. (both motors would need to be mounted to something very stable or you will have a multi tester helicopter flying around the room). You could then see how many volts are generated by the motor on the tester side. This would give some real world data to work with and hopefully show how efficient the system would perform as a generator.