Miraculously I found the answer I need for the exact model of motor control board I have. I want to thank you all for your suggestions and review the options in case someone else is in my spot in the future:
1) Ditch the controller and step it down with sheaves or gears: This was BeardedBrews response and certainly would be easiest, though the most costly. Large sheaves aren't cheap anymore. I'd need about a 13" sheave on the mill to step down a 1760 RPM motor with a 1.5" diameter sheave press fit to it. That's an expensive sheave, and there are other issues with this. A gearing system would be even more expensive.
2) Buy pre-built electronics to send signals to the motor control board: This is probably the easiest, and middle cost option. The trick is that the input voltage is 9V and the PWM signal voltage needs to be 5V with a low pulse duty cycle to bring it down to below 4V. This appears to be solvable by purchasing a 9V PWM DC motor speed controller and running the output through a voltage regulator set to 4-5V (or a voltage reducer, as it's sometimes called). That whole setup can be had on Amazon for probably around $20-30. Note that I haven't seen anyone actually use this setup, but in theory it should work fine.
3) Wire up a purpose-fit PWM controller: This website has the full gory details:
http://el34world.com/Misc/Cnc/TreadmillMotor1.htm . Now, I'm not an electrical engineer but I've worked with EEs a long time and picked up a few things along the way. I've also been itching to get my hands dirty with some solder (not literally). For me this is the way I'm going to go. I found all the parts I need online for about $30 after shipping, but if there was an electronics store anywhere within 10 miles of me I could have gotten the parts for WAYYYY cheaper. In any case, the parts are ordered and when they get here I'll try to wire everything up and see if I can't get this going. For me another bonus is that this unit will be compact (just the PWM speed controller and the motor control board) and can easier fit in a small 8"x10" hobby box. Less space, less mess, and less to lug around on brew day.
4) Use the treadmill panel to control the motor board: Certainly the cheapest option, since I have all of the parts from the treadmill. Also the least likely to give me the control I need. The lowest setting on the treadmill speed still has the mill rolling about 300 rpms. Not terrible, but not ideal. Also, the panel is big, like 2.5 ft square big. Mounting that to the mill board will require a really long plank and take up a ton of space, as well as being awkward to carry around. I wouldn't want to go this route unless I really had no other option.
So there we go. Thanks again for the suggestions. And we'll see how the PWM controller wiring goes.
Cheers!
