Have you tried just shorting the "power good" line and ground at the ATX MB power connector?
It should be the green wire. What I have successfully done in the past was, without cutting through the wires, strip about 1/2" of insulation from the green wire and an adjacent black wire, solder or twist them together, and tape them up well.
The result is a computer that you basically can't turn off....ever. Not sure if it works on all systems, but it has worked well where I have needed it.
RandyT