Another reference (thanks Sherry Haibara for sharing with me) to deliberately boot USB MSC is 'Booting your iBook G4 from a USB stick' by Ben Collins.

How to tell if your PowerPC-based Mac "officially" supports booting from USB storage: enter Open Firmware shell (hold command + option + o + f), run devalias. Is ud in devalias output ? yes, then it ought to boot from USB : no, then it does not easily boot from USB.

I have yet to try deliberately telling Open Firmware to boot from USB (I am very lazy). I believe, TiBooks boot from USB, then AlBooks removed that functionality (no ud in OF), then later Apple returned that functionality (around time of PowerBook G4 12-inch 1.3 PowerBook6,4).