Using ARC firmware instead of Openfirmware may have better results.
“What developer Rairii – who calls himself Wack0 on Github – has done is ported a version of the PowerPC ARC firmware, plus a loader to get it into RAM, and some basic drivers to bring up keyboard, mouse, IDE and a framebuffer to certain models of Apple gear: the tray-loading iMac G3, PowerMac G3 “Blue and White”, PowerBook G3 “Lombard”, and PowerMac G4 “Yikes”. This is a remarkable achievement: while he’s taken code from several other projects, including OpenBIOS and Coreboot, this is still a hugely impressive effort.”
I posted links to build ARC firmware: