Leopard will run on the AIO with a G4 upgrade.I remember reading Leopard would be capable on a B&W with a G4 upgrade. Do you think you could get Leopard on an AIO with a G4 upgrade?
Just sayin'.
You will need to disable the onboard video and replace it with a PCI Mac ROM video card (such as the ATI Radeon Mac Edition), then copy the Gossamer kexts into the system installation. Finally, use XPostFacto to boot up off that disc and into Mac OS X Leopard.
I have the Gossamer, B&W and Yikes kernel extensions here, was planning to experiment rolling them into LeopardAssist 3 along with the XPostFacto stuff. It never came to be, but with the files it can still be done, it's just a little more hacking.