From everything I've tried and read, G4 iBooks can indeed be reworked to boot into 9. The problem is that hardware drivers (namely, video card drivers and IIRC power/energy management) don't play nice, making them pretty much useless.
A really good start would be a working OS9 driver for the Radeon 9200 and 9550/9600 graphics cards. Open source drivers exist for x86 systems, but maybe porting those to OS9 would be manageable for someone with a little programming skill. I don't know about power management, but without those drivers the system runs at 100% and gets really hot.
And while we are dreaming, we may as well throw some Airport Extreme OS9 drivers into the mix.