The installer is unable to install Yaboot on the USB disk as it probably misses the code of finding the openfirmware path to that machine. The only thing that I can advise you, is to install yaboot manually. This is not easy for noobs. More info here:
http://mac.linux.be/content/yabootI would use a Debian live CD and from the live environment try to install yaboot in the Apple bootstrap partition on the USB disk.
http://live.debian.net/archive/images/6 ... werpc/iso/Booting such USB drives:
I don't know whether Apple will boot with holding option at boot. If that would not work, you would have to boot from open firmware:
http://mac.linux.be/content/booting-open-firmwareAll in all, not an easy project for Linux noobs....