I recently wrote
photo-db-fs a program that exposes the tags and ratings stored in the digiKam database as a FUSE file system.
My main starting motivation was so I could have my desktop background just cycle through all my favorite photos automatically. Since writing it I have found some other uses for it like pushing some tags photos to google photos via
rclone to show on the Chromecast background on my TV.
[anitschk@localhost ~]$ mkdir /tmp/myPhotos [anitschk@localhost ~]$ photo-db-fs \ --mount-point /tmp/myPhotos \ --db-type digikam-sqlite \ --db-source ~/Pictures/digikam4.db &  12910 [anitschk@localhost ~]$ ls /tmp/myPhotos/tags/DesktopBackground/photos 009318790e574d9764679ec1b8f0a987.jpg 5c23b47abdb881acee6b1f5313ecbc71.JPG 01586b0bc31424ab3b156ffdac8ef58f.JPG 5c4918818f3f14575f76c78ad1cf62d6.JPG 0338a700e5e602c496abdcad2deaa133.jpg 5da619a984339d0e1bd4b80de91a79cd.jpg [anitschk@localhost ~]$ kill %1
Hope some other folks might find this tool useful too.