I cull & rate images in Fastrawviewer, then process them in Rawtherapee. Is there a way of showing FRV’s XMP ratings in RT?
No, there is not.
That seems to be true. But…
There’s this mysterious entry in RawPedia’s Features section:
Rate photos using a 0-5 star system (ratings are read from embedded Exif and XMP), tag them by color, filter by filename and metadata.
Never been able to figure out how to get this to work, though. Maybe one of the RT devs can enlighten us on that one 
My guess is that RT reads ratings from embedded XMP only. It doesn’t read from xmp sidecar files that FastRawViewer generates.
It’s a kluge but I have a batch file that takes a list of file names and creates pp3 files for those file names, with the pp3 files having a desired hardcoded rating. I suppose a more elaborate batch file or script could read the xmp sidecar files, extract the rating and then create (or modify) the corresponding pp3 files to give the photos the same rating there.