As you wrote, the PCI-e G5 PowerMacs require a special "runway" card that enables mounting of standard AirPort/Bluetooth combo module known from Mac Mini/late PowerBook. Then you just hook up the internal antenna cables and you're good to go. Unfortunately, the "runway" adapter is hard to find and pricey like hell. You can mount an A1027 card though and it should work (or so I heard) but it would require a different antenna (some use it with antenna cable from "AGP" PowerMac G5 and an external one). Not sure about the standalone Bluetooth module though.