Searching through my CD archive I've found two different versions of the firmware updaters for the PowerMac G4 and the Cube (as well as the iMac, iBook, PowerBook and even a G3) on Mac OS install CDs:
|(both are still current)|
Using them I successfully downgraded the firmware of a Sawtooth from the current 4.2.8 to the older 4.1.8 and back to 4.2.8 again. Further analysis of the code shows that it will only flash a firmware with the same major version as in the ROM (or upgrade from 3 to 4 as a special case). Even the firmware from another model would be accepted — Never ever try such a crossflash as it will most likely leave the motherboard completely dead and therefore without a possibility to correct the flash !
The firmware NVRAM variables will survive the flash, including any NVRAMRC patches (as the lba-48 property), but do NOT flash a plain firmware with a (7447 or 7448 based) CPU upgrade that requires its own patches.
While at it I also tested manually booting the flasher from the Open Firmware command prompt. With one of the CDs from the table above (or a CD-R with just the "Firmware" file) it is possible to upgrade or reflash the firmware without any Mac OS on the system !
No Macintosh was harmed during the making of this post.