I've had my first 2nd-hand look at a WMP54G. It appears that it is a different PCI device, perversely enough. So the thing to do is download the perl script, then change the obvious spot near the top from "pci1737,4320" to "pci1737,13"
Also, after running the perl script, you may need to do this:
cd /System/Library/Extensions
kextunload AppleAirPort2.kext
kextload -t AppleAirPort2.kext