..(PWR_OK) but I cannot find any references to what this pin does or how I could override that function.
The way I understand PWR_OK... it's a signal furnished by the PSU (a +5V logic "high") to the motherboard that says "power is good and stable now". That's when the motherboard actually turns 'on'. You could put a peak-reading multimeter on that to see if it ever enables. But a PSU with decent enough OCP might never even turn on if it sees a short circuit on the +12V output so it would never enable.
To see if your PSU uses it, with the PSU disconnected from the motherboard put the multimeter on PWR_OK of the 24 pin and see if it goes HIGH after shorting 15-16 to turn it on.
From WikiPedia:
"The ATX specification defines the Power-Good signal as a +5-volt (V) signal generated in the power supply when it has passed its internal self-tests and the outputs have stabilized. This normally takes between 0.1 and 0.5 seconds after the power supply is switched on. The signal is then sent to the motherboard, where it is received by the processor timer chip that controls the reset line to the processor.
The ATX specification requires that the power-good signal ("PWR_OK") go high no sooner than 100 ms after the power rails have stabilized, and remain high for 16 ms after loss of AC power, and fall (to less than 0.4 V) at least 1 ms before the power rails fall out of specification (to 95% of their nominal value).
Cheaper and/or lower quality power supplies do not follow the ATX specification of a separate monitoring circuit; they instead wire the power good output to one of the 5 V lines. This means the processor will never reset given bad power unless the 5 V line drops low enough to turn off the trigger, which could be too low for proper operation."
So, if you have a cheap PSU you might need a more elaborate setup than a peak-reading MM to determine proper functioning of the PWR_OK line, but you'll at least see if the PSU is providing a +5V output there to turn on the MB.