I tested some of the motors at high-speed to see how much they'd generate and to see if any would be suitable. Note that the motor speed was dictated by the shaft diameter; the drill motor, 6V motor, and original motor had larger shafts and ran slower.
@ 3 ohms
@ 3 ohms
For the stepper motor, at 1,100 rpm, using both phases through a bridge rectifier, it would source 0.63V into 3 ohms or 0.13 watts. One phase through a 5:1 step-up transformer frustratingly yielded the same results: 0.6V. Both phases in series yielded 0.67 volts, and at 1,720 RPM, 0.8 volts. The open voltage on the transformer secondary was 11.75 volts.
I also took apart one of the pumps that I used to use for the air horns in the car and got its motor separated. It could source 3 volts into 3 ohms and had a short-circuit current of 5 amps at 3,100 RPM.