I wrote a plugin for munin to graph the active users in Subsonic (users currently playing music/video) : you can find it here
Just add it in your /etc/munin/plugins directory and don't forget to set the URL, LOGIN, PASSWORD options (login/password of a existing account in your subsonic)
I personally created an account for Munin with no right (can't even play music).
After few minutes you should see something like that :

The plugin is working with version 4.6 (build 2585), and I have no idea if it can work with any previous version.
Cheers.
[EDIT]
v1.1 : Updated the parsing of the JSESSIONID
v1.2 : Automaticaly delete the cookie
v1.3 : Use the API