As a cross-platform user myself, I have some ideas:
Check out
http://www.versiontracker.com/dyn/moreinfo/macosx/17254 for a utility to export your iTunes playlists as m3u files (Winamp playlists). The m3u files will work in MediaCenter.
Check out
http://www.panic.com/audion/ for a Macintosh MP3 player that may be more capable than iTunes in your ratings search.
An interesting possibility is that ratings may transfer from a Windows-based iPod to Macintosh iTunes. I will attempt this in the near future--once I get the newest beta of MC9.1 working with my iPod.
Mostly, however, I just use my Windows jukebox as a repository and use OSX to connect to my MP3 drive as a Samba share. I then imported all the MP3 files into iTunes, leaving them on the Windows drive.
That has left me with a local iTunes library that points at my shared Windows drive. To listen to, rate, and otherwise manipulate the tunes, I just make sure that the shared drive is connected to my PowerBook.
The drawback in this situation is that the MC library and the iTunes library will never be in synch unless you make all changes to both libraries.
Hopefully this will spark some ideas....
Best,
Brad