MPRIS2is becoming the standard way of controlling media players over DBUS. Since GNOME MPlayer already had much of this capability built in due to the usage of DBUS to communicate with gecko-mediaplayer I decided to add MPRIS support in to the upcoming 1.0.7 release of gnome-mplayer.
After a few days of work, much of the MPRIS specification has been implemented. Really not that hard except for having to figure out how to implement some of the more unusual types. The mpristester application was a huge help in debugging and ensuring that the implementation went correctly.
After initial support was working I began testing the code with a few of the gnome-shell extensions that use MPRIS to control several media players. I had to make a few modifications to them, mainly just adding "gnome-mplayer" to the list of supported players and after that they just started working.
So if you use one of these extensions, tell them to add support for gnome-mplayer to there application. And as always if they have issues, I should be able to fix the problems within a short period.
The code is in SVN now, and I recommend that you get gmtk and gnome-mplayer from SVN for the best support.