I haven't been reading this overly closely so I may have missed something, but figured I'd just toss this out there: If this process depends on hacking the original Apple firmware upgrade file this may do bad things if you try running this on a B&W upgraded to a G4 ZIF. One of the things the firmware update did (to substantial outcry) was add a test to see if the user had upgraded to a G4, and if so to halt the machine before booting.

If you actually want to make this something you distribute I might humbly suggest looking at the "Blue & White G4 Enabler" from Newertech:

http://eshop.macsales.com/tech_center/software.cfm

I'm fairly certain the way *it* works is by copying the existing firmware from flash, hacking around the G4 test, and flashing it back. You might want to do the same (IE, copy whatever firmware is already on the G3, hacking the startup sound, and writing it back) to avoid the edge case of bricking someone's G4-upgraded B&W. (Which might be a real problem if they didn't save the G3 ZIF when they upgraded.)