I concur with Chris about reducing voltage.  The right way to do this would be with the full voltage and PWM (pulse width modulation) for the speed.  That way full torque is available but speed will vary.   In my past, I've done troubleshooting work on repairing the circuit boards on Cessna autopilots and that is exactly how they operate.  More importantly, if anyone uses such a device at altitude, the full voltage will prevent motor stall from the air gap thinning out.   Many of the older trim systems with dirty brushes and rotors fail at altitude but work perfectly fine on the ground for that reason.  If someone builds it, I would add it or try it or buy it.  Some additional comments on trim from the Cessna autopilot:
1) Trim is set to run faster up than down by about 20% for runaway and autopilot purposes
2) Airspeed controls the dual speed setting
3) on the C414A it took 32 seconds for full up to full down but that number has a lot to do with control if the trim starts to runaway.  My Legacy is 10 seconds and that is too fast.

If anyone wants the 28v circuit for such a system, I have the old manuals somewhere and the parts are simple discreet components with a 1000 Hz oscillator for PWM frequency.    

Great thread, hope something comes of it.

Paul Miller
L2K