It strikes me as the sort of thing which is easy if you know about these things. So you need to find someone who knows about these things. I would ask Tycho. He knows all about these things. Don't expect a quick reply though, he's a busy man these days.

If it helps, the G4 PCI Graphics (aka Yikes) uses the same board (and hence PSU) as the B&W G3. Maybe that info will help you find a previous example of someone doing something similar. Your PSU is actually very similar to a stock ATX PSU. It has an extra 5V line (think its -5V, not sure). Its usually white.

You can easily convert it to ATX by snipping the white 5V wire. This means if you can find a faulty logic board from a G4 PCI, G3 B&W, or any ATX PC (or Beige G3 they are ATX too), then you might be able to use it switch the PSU on. Its not an elegant solution, and you'll need a CPU too I guess, but it should work. Perhaps you can find a cheap board with a broken port or RAM slot.