Did you ever try this out? I made a similar PCI-E plug converter mod for one of these machines. I found I had to have all the 12V lines plugged into the same PSU rail in order for it to work, but I didn't want to run it too long like that since the wiring for the cheap power supply I was using looked awful thin.
I did get it to work just fine. Initially I tried a new power supply I had on hand - but had intermittent luck (using a jumper) getting it to power up since new supplies are switched via the main cable to the motherboard (some sort of soft switch).
I then tried a much older power supply that had a mechanical power on switch. Works fine now. The power draw doesn't seem a problem - not sure if this old PS has more than one rail - it is a very old entry level Lite On. Hope that is of some use. I am sure someone else can give a better answer.