So I'm dual-booting Ubuntu 12.04.1 for powerpc64 alongside my Mac OS X on an iMac G5.
How do I enter single user mode now that I have yaboot installed? Is there a command from openfirmware?
I read somewhere on the internet that I have to set Mac OS X as the deafult OS in the yaboot.conf file and power down the iMac and hold whatever keys I need to hold on the keyboard and just wait until yaboot boots to Mac OS X by default. Just add defaultos=macosx to your yaboot.conf located in /etc/yaboot.conf. You can do that by starting the terminal and authenticating as root by typing "sudo su" and then typing "gedit /etc/yaboot.conf" which will open a text editor where you can add that line after the lines you have, but before the "image" line break. Typically you should add it after the "enablecdboot" line.
I haven't tried this yet as permissions on my iMac were totally screwed, but I think it should work!