It seems like a 6RPM Geared DC motor will do the job, it's available from the internet, it's speed depends on the input voltage, and it makes the control circuit a very simple thing, and so making it to use with a ST-4 style control would be asy.
So, the only problem now is to really buy one, and to mount it on the place of the slow motion knobs.
Seems not a hard project, let's see. It's definitely a high priority project when I've time.
To shop list:-
1. shaft coupler: to connect the motor with the slow motion control knob shaft (~6mm)
2. solid state relay: to control the on-and-off of the motor, maybe it's not required? can be powered via the ST-4 signal? is it electrically safe?
3. of course, the motors