I found a dirt simple approach, no scripts:
1. Rate your music via the normal Plex clients a. I use a 1 to 5 Star scheme
b. I rate as I listen, or not
2. Create Smart Playlists on Plex Media Server a. See:
https://www.ryananddebi.com/2019/08/26/plex-how-to-create-smart-auto-updating-music-playlists/ b. I created (5) playlists as: 1-Star, 2-Star, 3-Star, 4-Star, 5-Star
c. You can add additional Plex library criteria as you see fit
d. These playlists are always refreshing based on your criteria/actions
3. Add the "ExportTools" Plugin to Plex a. See:
https://awesomeopensource.com/project/ukdtom/ExportTools.bundle b. Essentially copy & paste the "bundle" into the correct Plex location
c. Here: %LOCALAPPDATA%\Plex Media Server\Plug-ins
4. Using the “ExportTools” Plugin from within Plex a. Here: Plex Settings > Manage > Plugins
b. ExportTools Settings > Select Playlist > Reload Playlists > Save (first time thru only)
c. For each Playlist: Settings > 1-Star > Create m3u8 Playlist > Simple > Save
d. The (5) playlists (both csv & m3u8 formats) are written to the “...\Export\ExportTools\” folder
5. Using JRiver Media Center, Import each Playlist a. Import each of the (5) Playlists, one at a time
b. File > Import Playlist > “Playlist – 1-Star…m3u8”
6. From JRiver Media Center, Modify the Ratings value a. Select the desired Imported Playlist (i.e., 1-Star)
b. Select all the matching tracks (Ctl_A)
c. Select 1 Star Rating (modifies all the selected tracks at once)
d. Rinse and repeat for each of the 5 Playlists
e. Done!
For my 1500 track library, I was able to complete the Export/Import/Update process in less than a few minutes. Easy!
I plan to repeat the process every now and then to keep Plex and Media Center Ratings somewhat in-sync.