The firewire ports work fine after startup but they are not bootable unless they are fed by the +28Vsb line that is always hot. ATX PSUs don't have a +28V line, their +5Vsb line isn't enough to boot firewire drives. The front panel on the B&W steps up 12V to 28V for the Smurf's firewire ports (on G4s this function was taken over by the Mac PSU). So a B&W front power panel can be spliced into an ATX PSU to make the equivalent of a Mac PSU. It steps the +5Vsb up to +14Vsb which is enough to boot from firewire drives (still not enough for ADC monitors, you will need to use a PC monitor). I found the directions here:
http://forums.macrumors.com/showthread.php?t=261623