That's super awesome of you. I just now (literally) got it to boot with the G4 upgrade. The firmware updater I found on Sonnet's site ran under OS X. It installed two scripts in the Applications folder: one to install the patch and one to reverse it. The way this one works is you hold the programmer's button and power on the Mac. Once the long beep is heard, you let go of the programmer's button. The Mac seems to reboot after the long tone, and boots into OS X. You then navigate to the Applications and run the installer script. It prompts for an admin password and then patches the firmware. Once done, it gives you a dialog indicating whether or not it was successful. In my case, it was. After you install the patch, you then install the CPU upgrade.
For some reason, initially, my Mac would not boot after the firmware was upgraded. I reinstalled the firmware, shut the computer down, installed the CPU, removed the PRAM battery, waited a few minutes, reinstalled the PRAM battery, and turned the computer on. At first, it acted as if it wasn't going to boot. So, I did CMD-OPT-P-R to reset the PRAM. The Mac did not appear to respond to the key presses. So, I just held the power button until it turned off. Waited a few seconds, and powered the Mac on again, this time to hear it chime and boot. Victory! It's too soon to really say, but it appears to be stable. I have no explanation for why the computer would not initially boot. Those of you with more expertise, feel free to chime in.