Woah. Since posting this, I found like a TON of forum chatter out there. People seem to be doing this right and left. I actually have the rudiments of a plan forming. If I decide to gut my reliable old Mac and replace the innards with pure ATX evil, I'll totally post it all here.

So far, I've got a GA-EP35-DS4 mobo, a GeForce 8800GT, and a Q6600 SLACR cobbled into an old Dell PowerEdge 1500SC enclosure, and able to boot as far as the Chameleon loader on a 1GB USB stick. So far, I'm impressed and encouraged.