If you are still looking at solving this I came across a tidbit, but I don’t have iTunes so I can’t verify that it works.
You can create a playlist of all your 5-star tracks and export it. You will then have a list of tracks that you could set to 5-stars in the flacs, if you can find an MP3 editor that can do it in an automated way.
Repeat the process with 4-stars, etc. Not the most elegant but might change the impossible to possible.