mpd-add-similar is a little cli app that can populate your current MPD playlist with similar tracks to the ones in it. Using it is as easy as typing `mpd-add-similar` in a terminal. Run `mpd-add-similar -h` to see how you can tweak it's behaviour.
A tarball is available at the project's page as well as a link to an Ubuntu ppa.
See also: Playlistbuilder