I’ve just finished work on (the first version of) my specialised XMMS2 client, MKeys. As you might guess from the name, this client does one thing, and one thing only.
MKeys is designed to sit quietly in the background, and whenever you press a multimedia key (configured in GNOME) it captures it (much the same way as Banshee/Rhythmbox et al do), and instructs XMMS2 to take the appropriate action.
I like to think it’s pretty simple to use, all you really need to do is run it when XMMS2 is running (although it’s usually nicer to put a script to run it in your startup.d folder for XMMS2). It will automatically exit when XMMS2 closes.
For those who want to give it a whirl, the source code, Git repo, and tarballs of v0.1 are available at the project page on github: http://github.com/DarkOtter/MKeys