Alright, so I've made pretty significant progress in this hack.
I've dropped a DA board into a highly modified PowerMac 8600 tower case, picked up a QuickSilver 933 processor, and re-wired an MDD power supply to match the DA G4 motherboard as per XLR8 Your Mac and http://www.outofspec.com/frankenmac/wire.shtml.
I know the DA board works because I ran it in my DA G4 case (with DA G4 PSU) after I bought the board to make sure it works. No problems there. I don't have an MDD to test the PSU on, but it was sold as a working PSU...
When I power on the DA (either by shorting pins 3 and 6 on the front panel connector or by installing a QuickSilver's front panel board and pushing the power button), the light lights up for about 1/2 second, the PSU comes on, and the red LED next to the RAM slots on the motherboard lights up. The PSU fans stay on and the LED stays on, but I don't get a chime, and the power LED goes out. There are no diagnostic LED flashes or speaker beeps. I tried swapping in the original 466 MHz DA G4 processor board just in case, and that didn't make a difference. I'm using a known good 512 MB SDRAM DIMM (works in my other DA G4), but taking it out doesn't affect the outcome any (I don't get the tones for missing RAM - I still get nothing at all).
I tried the same board in a QuickSilver G4 of unknown status (either the QS board or QS power supply is faulty - I'll be posting a separate thread about that) after disconnecting pin 9 on the PSU (the extra ~28v which is not connected on the DA G4). I get the same results.
Anybody have any idea of what's going on here? I'm at wits end (and a few hundred dollars poorer for all the parts)...