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/yaboot
I 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-firmware
All in all, not an easy project for Linux noobs....