I glued the motor/gearbox board to the front of the existing barndoor mount (after removing the stepper motor and stepper motor control board).
Unfortunately, after re-testing, I found that the motor turns at around 1.28 RPM (45 - 47 seconds per rotation) using a 1.5V AA battery and a 1N5401 diode for dropping the voltage (the 1N4148 blew up, not beefy enough).
I was able to get another bolt, with around 33 TPI (threads per inch), as compared to the old bolt which had 20 TPI.
At 1 RPM and 20 TPI, the linear movement is 0.05 inches per minute. At 1.28 RPM and 33 TPI, linear movement is 0.039 inches per minute. There's quite a bit of error, but I'll see if the tracking is good enough for 30-second exposures.
I've verified that even with a worse-case load (200mm lens pointed directly over the motor drive), there is no slipping.
I also purchased a Slik Mini-Pro III tripod (with ball-head) for 1,100 pesos at Camerahaus. The 400-peso mini-tripod from KL didn't have enough holding torque on its tiny little ball. With a "real" ball head, things are much more secure.
Now the proof is in the photography.