I usually have Xmms minimized and off the screen 99% of the time since it always has the highest desktop priority and gets in the way of everything.

You just introduced me to xmms-ctrl the other day so I don't know what all commands and features this supports.

I was just thinking of a full screen GUI, and when clicking on a certain area, this would send a specific xmms-ctrl command.

To me, this might be the very simple way to go.