My contact from Precision Micro-drives was kind enough to send me a drawing and a link to explain how he would do it using MOSFET, however I'm afraid this is out of my league and I'm in need of someone to explain how all the components are working and why etc.
Haven't we already established that the motors don't support pwm?
The datasheet does state that but in the performance graph the voltage varies between 1.6 and 3.6 to show it's characteristics.
In the drawing provided the signal into the mosfet does infer it though. The OP did not provide the "link" so can it?
I would be tempted to try if I had a few extra . What happens to it if you do pwm it?
Got a pot to do a simple varying voltage setup? Does it work? Control the pot with a servo or get a digital pot.