Comment 7 for bug 502474

At http://muth.org/Robert/Hacks/ there is a shell script that can help:
 rhythmbox-tool.sh "Miscallenous tools for exporting and backing up data (playlists, ratings) for rhythmbox."

It is out-of-date in terms of where to look for the database, but seems useful anyway. E.g. this command will export ratings in the format "rating file-url" if you have installed the xmlstarlet command:

xmlstarlet sel -T -t -m "/rhythmdb/entry/rating" -s D:N:- '.' -v "concat(.,' ', ../location)" -n $HOME/.local/share/rhythmbox/rhythmdb.xml