In Jaguar it is a lot easier. You simply need to take your Mac OS 9 keyboard script and add a .rsrc extension to it and then drop it in the /Library -> Keyboard Layouts or ~/Library -> Keyboard Layouts folder. Log out, log back in and it all works.
So, if you need to add another keyboard script to the system that Apple doesn't provide (Russian phonetic/transliterated in my case), you can do it without any problems.

