I wanted to put this info here because we often edit the nvramrc so we can boot OS 9 on unsupported models.
If you corrupt the nvram with the wrong code, it can leave your Mac unbootable and Command+Opt+P+R doesn't work.
Here is what you do:
For desktops, just hold the power button, and keep holding......You'll hear the programers tone, keep holding, you'll hear a boot chime, then the system will enter Open Firmware with the defaults loaded. Now just reset the nvram.
For Portables, hold Power and Apple Key, keep holding, you'll here the programers tone, keep holding, you'll hear a boot chime, keep holding, the system will enter Open Firmware and you can reset the nvram.
This will likely work to bypass an Open Firmware password.
I'll test it and get back to you.