We don't sync back so I haven't tested it, but as long as you have a gallery that writes back to a sidecar file, digikam can work with it.
Digikam has direct integration with Exiftool as well as batching, so anything you can get in to a sidecar file, you can get out with Digikam, and then stick in custom albums/categories etc.
Some quick searching turns up a tool called Piwigo (which I haven't tried) that has the ability to run contest modes via plugins. Assuming it can write the votes/results back to the sidecar file in some manner, you should be able to get to it in Digikam