I've made progress on this. I found that the Delta power supplies used in the PM 8500 do in fact have a fan speed controller. The fan's + is connected to +12V, but its ground doesn't go to ground. I also found the thermistor nearby (lower resistance = higher temperature). I connected two wires to it and brought them outside the case. With a 100K variable resistor across it, I am able to speed up the fan and make it hit that resonance frequency. Interestingly, it can be made to run a lot faster than normal (I found the same when I plugged the fan into straight 12V; it moves lots of air). Apple made such a big deal about the variable-speed cooling fans in much later PowerMacs, I didn't expect such a thing to be in a "lowly" PM8500. For some reason I find this really surprising.

I'm thinking my simple solution for now will be variable resistor with a toggle switch across the thermistor, with the resistor set to slightly speed up the fan. Then when the fan starts to make that awful whine, I'll just toggle the switch, which will either speed it up slightly or slow it down, depending on whether it was closed beforehand. Either way it will get away from the resonance speed and allow me to stay sane.