Love the idea. Please describe how to hook up applications to receive such notifications. I'd love to have MirrorManager's update-master-directory-list cronjob listen for such instead.
I don't have any hands-on experience with Fedora Message Bus. But we can ask the Messaging SIG guys [1]. On their mindmap they even have MirrorManager mentioned as one of their use cases.
My naive idea is that there would be a simple script (Python or whatever the qpid library has bindings for) that would connect to the bus and wait for the user-defined event, then trigger a user-defined command. We could distribute the script as part of the MirrorManager toolkit. I don't have any MirrorManager experience either, so maybe this could be easily integrated into the existing set of scripts.
This is basically why I propose this, to learn more about the topic and see whether there is someone willing to go ahead and try to implement this.