A little googling found this:

http://pragmattica.wordpress.com/2008/01/17/convert-itunes-m4a-files-to-mp3-on-linux/

...it kinda seems that, given storage is so cheap, you should be able to hack some sort of script like that up to give you a totally up to date mp3 mirror of a mixed (or aac only) iTunes library.

Then you just need to get it onto the empeg with mp3tofid, I guess...