I haven't grabbed a voltmeter yet to figure out which is which, but could we just jump the speed signal wire with the hot wire and make the fan run full speed all the time?
If you cross the +V with tachometer you will get the maximum reading of 255. Since the fans read rpm in multiples of 120, 255 * 120 = 30600 RPM.
At least with the latest version of the firmware (Nov 2017), the software understands that the tachometer has shorted, and will stop mining if either fan has the 30600 rpm speed.