on trac.fireflymediserver.org you find a detailed API doc for mt-daapd.
Yeah, such an applet would rock. It should be able to monitor remote instances of firefly too, if it follows unix philosophy.
I’m in the process of getting my build chain for the NSLU2 up and running – its far easier to plain compile for Linux.
I’d gladly assist you with a panel-applet. I do have zero experience in writing panel-applets though 🙂