and the user library which is on the internal HD, regardless of where the actual music files are.

When you plugged in the MacBook, none of that information was available to iTunes.

Will you be getting the iMac G5 hard drive replaced? If so, then the first thing you do when you turn it on is to do a Restore from your Time Machine backup.
This will replace all the information that you are currently missing.

Once that is done, you can look into moving the iTunes information over to the MacBook

P