I've always been under the impression that it's generically unsupported to boot OS X on any PPC machine over USB, even though the older machines could boot OS 9. (Once upon a time I successfully pared down a 9.2.2 system file to fit on a 64MB USB key and booted my 867Mhz Titanium with it, just for shnitz and giggles.)
That said I have *heard* of it being done (At least on G5s and Aluminum G4 Powerbooks), but I think one of the keys to all the accounts is it's only possible to fake the machines into booting an existing "working" OS X partition cloned en-mass to a USB drive. You talk about "loading the installer" when booting from the attached drive... what are you doing, attempting to boot an OS X CD/DVD install image DD'ed onto a disk? (And it is possible that the success stories may well only apply to newer machines/laptops. Apple used several different motherboard chipsets in the late-G4 era, and the DDR-memory-equipped PowerMacs used several confusing variants with different capabilities.)
That said I have *heard* of it being done (At least on G5s and Aluminum G4 Powerbooks), but I think one of the keys to all the accounts is it's only possible to fake the machines into booting an existing "working" OS X partition cloned en-mass to a USB drive. You talk about "loading the installer" when booting from the attached drive... what are you doing, attempting to boot an OS X CD/DVD install image DD'ed onto a disk? (And it is possible that the success stories may well only apply to newer machines/laptops. Apple used several different motherboard chipsets in the late-G4 era, and the DDR-memory-equipped PowerMacs used several confusing variants with different capabilities.)