HAS commented that I could track changes to the player state of iTunes using NSDistributedNotificationCenter.
I’ve gone ahead and made changes to the iTunesBriged code to show how to do that. Now, whenever iTunes changes state, whether it’s via the iTunesBridged application or by directly manipulating iTunes, the title of the Play/Pause button changes and the current track name is listed in the text box.
As mentioned in that comment, Notification Watcher is a great app for observing distributed notifications. It made it really easy to figure out what iTunes was sending and what data was in the userInfo.
Thanks for the comment, and the idea!
Get iTunesBridged source downloads, which can be cloned with Mercurial or downloaded as a tarball.