Is there a way to subscribe to messaging menu events? Specifically, I'm looking for a signal (over DBus?) that is fired when the online presence status changes in the messaging menu
The messaging menu uses the Telepathy framework and sends the status events to Telepathy's Mission Control. The webpage that describes Telepathy and how it works is here:
If you use the Telepathy-GLib library you're probably looking for the "most-available-presence-changed" signal from the TpAccountManager object.