I just posted on another topic regarding Flacs and RB DJ Export mode. Rekordbox (RB) Does support FLACs but can't read all Tags. Whenever I add a FLAC track to my collection I always have to manually input the missing tags. Luckily Comments come across, so for the Tags I know RB DJ doesn't natively read, I add them to the Comments from a tagger like MP3Tag. For instance you can have MP3Tag write any fields into your comments by using a script.
In your case you could write the RB Star value into the comments field in RB so that you could pick it up in Foobar. Note Rekordbox does store the Tag locally.
It's not ideal but at least when you import the file into RB or FooBar you have the information available.
So you update your Tags in Mp3tag or FooBar (which also does scripting I believe), and then run the script on your FLACs to update your Flac Track Comments. When you bring the Track into RB, you simply take the fields from the Flac Comments and place them in the corresponding RB Track Fields. If you edited the track outside of Rekordbox remember to re-import the tags (right click the track in your collection).
Note to go the other way, most of the RB tagging information is exported to the Rekordbox.XML file, so if you wanted to automate FLAC tagging from RB you could use the Rekordbox.xml file to help. Works great and here is where you'll find your rating!
This is the Track Header information in the Rekordbox.Xml file:
<TRACK Name="" Mix="" Label="" Tonality=""
Remixer="" Location="file://" Rating="" PlayCount=""
Comments="" SampleRate="" BitRate="" DateAdded=""
AverageBpm="" Year="" TrackNumber="" DiscNumber=""
TotalTime="" Size="" Kind="MP3 File" Genre="Dance"
Grouping="" Album="" Composer="" Artist="" TrackID="1">
Again not ideal but I figured I would give you some extra information if you can use it!