Seems like we can build a shared library, libmpv, since mpv-0.4.0, and use that as the engine for a GUI. I managed to debianize and build packages for a QT5 front-end called baka-mplayer after backporting QT 5 5.3.1 to Debian Wheezy. It works well, but is missing many bells and whistles that we love in SMPlayer.
http://main.mepis-deb.org/mepiscr/testr ... a-mplayer/