Reading the MacAddict forums, I came across a thread called OS X Key Mappings. In that thread, there's an email posted from Marcel Bresnick, the author of "PrefEdit," who also has written some OS X Server 1.x keyboard mapping hacks. Here's a snippet of his response on customizing the OS X keyboard maps:
The keyboard layouts are stored in:He goes on to explain how to edit the files (in theory, not step-by-step). Head on over to MacAddict and read the rest of the Marcel's email for an overview of how the process would work. Warning - this editing is not for the timid, and if you mess up, you'll have no keyboard -- so back everything up first, and make sure you know how to use single-user mode!
/System/Library/Frameworks/Carbon.framework/Frameworks/
HIToolbox.framework/Resources/*.lproj/Localized.rsrc
The * has to be replaced by the respective language name. Every language package contains 32 keyboard maps for _all_ keyboards, so there are in fact 224 (!) keyboard definitions. (This doesn't make sense, looks like a quick hack to implement Macintosh keyboards on top of Darwin...) If you want to do it right, you would have to repeat the keyboard redefinition for all language packages.

