Yep - you have to manually edit your external yaboot.conf. Check this out from the archives where I put it on a small external firewire:
http://ubuntuforums.org/showthread.php?t=314237
You can use the "alternate" disk to get to a rescue prompt, just stop it at the second-stage boot: prompt, hit -tab- to stop the countdown and specify one of the options like
and get to a root shell in /dev/sda3
Hopefully there's something in that thread that'll get you up and running...
oops - you probably won't have to use the ofboot line anymore.. Don't forget to hold down the ALT/Option key upon boot to choose it after the ybin is successful..