It's completely doable to boot OS 9 on any G4 Apple ever made, I got in booting on my 1.5Ghz 12" Powerbook, it just takes a little learning and testing.
It just comes down to a matter of what works and what doesn't, USB trackpads on the latter iBooks and Powerbooks are an issue, they are not a HID device the OS 9 understands, even under OS X they won't work if you install an version unsupported on your Mac.
GeForce powered Powerbooks like my 12" won't have any graphics control 'NDRV', but ATI powered ones should have basic display support( 'NDRV' ) with a patch.
Sleep and power management won't work, and without an Open Firmware hack, 7447a powered iBooks and Powerbooks will run at 50% CPU speed under OS 9.
USB only works at 1.1 speeds.
Airport Extreme doesn't work.
Cardbus may not work.
Sound won't work on 2005 models.
Other than that, with the correct knowhow, you can boot OS 9 on any G4 Mac, just take the time to read some threads around here, and learn to build a custom Mac OS ROM.