Put a new (via epay) logic board in the Powermac G5 and got it running, hopefully for quite some time. Found the service manual and the Apple System Diagnostics disc online, so was able to run the required thermal calibration myself, instead of bothering the Apple store.
Turns out it wasn't the DIMM sockets, but the memory controller IC, which is a BGA package. Oh fun. It is mounted on the back of the board, and the heatgun was heating it up to get it to work temporarily. If anyone wants to school me on BGAs, feel free. I may try to fix the original board if I can get access to some shit at work. Was thinking about doing a reflow in the oven at home, ala some of the XBox or PS3 fixes you may have run across, but there is a lot of stuff that would need to be protected from the heat. Plus the no-clean Kester 951 flux I got is flammable. That and a gas oven made me pause a bit... honey, do you mind if I cook a board in the oven? Oh yeah, it may start on fire