Is there an existing MPD client which stores rating information in the MPD database. Plugins are acceptable.
EDIT It was pointed out in the comments that it was not obvious what MPD is.
MPD (https://www.musicpd.org/) is a music player that runs as a background process (usually on *nix). A MPD Client is a program that connects to MPD over the network to provide a user interface for MPD, there are clients for all major operating systems inkluding smartphones.