Ach, I was going to say, a workaround is to use the discs to boot the iBook, and mount the iMac with firewire target mode; you should then be able to install OS X onto the iMac that way. If it fails to boot up, well, fine.
Anyway target disk mode is in the firmware, so yeah, you can do it without the OS installed.
Sorry for the earlier misinformation.