Ok, so I haven't figured out power, I admit it, and that's why this isn't going anywhere right now; but I think I found something potentially useful to me, and I *think* Herma is working on helping me out...
http://www.ti.com/lit/ds/symlink/tl1451a.pdf
http://www.ti.com/product/tl1451a
http://www.ti.com/product/tl1451a-ep
I plan to use a 5v brick, and have a step-down, step-up, and a passthrough to power this thing. I think this chip could be helpful in that. The problem being that I'd have to build a circuit for it. In the end, unless I custom etch boards and use surface mount components, I think this would give me a power board that is around 2*2 inches. Decent, but I'd still like it to be smaller.
Why I think this would be useful:
Supports two output voltages from one chip
Locks power output until internal circuit is fully powered on, thus allowing me to use the 3.3V output as the PSOK signal
There's a lovely datasheet that has a schematic for me to plug numbers into on page 21
Basically, I'm thinking All I'd have to do to make this work is build the circuit, and then wire up a couple of transistors that control my 5v line with 3.3v: One ahead of the chip, to turn the chip on; and one behind the chip, to allow the 5V to pass on to the board.
If I'm being an idiot, please, let me know. I have minimal actual knowledge of what's going on here, I'm just going by what I think sounds right.
I've also done a little bit of work on the board... pulled off ports, wired directly into the power controller, moved a couple caps, etc.
Haven't gotten around to testing any of it yet, though, so for all I know, I have a fried xbox now.