The biggest problem is that iTunes only stores the ratings in the db itself (not in file tags).
Only real 'work around' is to move the files in to folder branches by rating and import one branch at a time. For example, if your current music is stored Artist\Album, you would change this to Rating\Artist\Album. Then import each 'rating' folder one at a time in to iTunes and add that rating to all of the unrated files. Once you are done, you can move the files - but I am not sure if iTunes will still see them as the same file. Test with a couple to start.
Another thought (that I haven't tested) would be to start with an empty iTunes db and set it to sync all files to the iPod. Shut down iTunes, open MC, sync to the iPod then disconnect the iPod and shut down MC. Now open iTunes and add all your music. Hopefully the next time you hook up the iPod, iTunes will see the 'changes' to the ratings and pull them down. Again - test with a couple of files (but this will probably require you to blank your iPod in the process).