I'm just trying to do the same thing.
I've manually added the device I'd to the Info.Plist file in the ATI4800Controller.kext (once I realised the info.plist file was hidden), on subsequent restart if has loaded the driver. (for reference I haven't installed the patches in Multibeast, so this is in the OOTB kext file)
my current issue is the Graphics performance is terrible, for instance clicking on Launchpad takes quite a few seconds to eventually paint the screen.
For reference the Information I added was 0x94601002 to the existing string values under IOPCIMatch
Let me know how you get on