@Chris C: SpeedFan actually lets me set the control mode (PWM or DC) and it's currently set to PWM. It also has a "reverse PWM logic" option. Sorry for thinking that it was voltage regulated and getting you running in a wrong direction . SpeedFan doesn't let my directly change "voltage" but instead lets me change the percentage of the fan speed. Reversing the PWM logic makes it so that 100% -> no fan spinning. So I guess we have PWM available even in 3 pin configurations. Good to know, sorry again for the inconvenience. I will look into the power mosfet solution.
Ok, this is good news! I believe it can be made to work simply with as little as two parts, like this:
1) Connect a 1k, 1/4W resistor between red and black on the motherboard fan connector. (Common, through-hole, 5% tolerance resistors are fine.)
2) The fans will be powered by the 12VDC/GND from a high-current Molex connector.
3) There will be a single N-MOSFET between the fans and ground, of sufficient power rating for the combined total current of the fans, and with a Vgs of 18V or higher. If a large enough MOSFET isn't readily available, several can be used in parallel.
4) The MOSFET gate(s) will be connected to either red or black on the motherboard fan connector. (This depends on whether the motherboard is switching the power or ground in PWM mode, of which I'm not certain. But one or the other will work, the parts remain the same either way, and hooking it up wrong should cause no harm.)
5) And finally, connect one (and ONLY one) yellow lead from a fan to the motherboard connector. Speedfan will then register the speed of that fan. There will be no way to read the speed of the other fans, unless you add a switch.
Experts please check this solution. I assume the fans won't be interfering with each other in any way that will require a separate MOSFET for each fan, but as I haven't done this in real life, I'm not 100% sure.