Hello!
I've been working on the MassMessage extension, and am planning on having it deployed on Wikimedia sites[1]. The extension allows a user to send a message to a list of users across all projects. It is intended to replace the current global message delivery[2] system we have. Any design input you might have would be appreciated.
The special page will only be visible to advanced users and administrators who are sending messages. I've set up a test wiki where you can play with it and test it out[3] (it might be a bit slow). You will need to login (username: 'Admin', password: 'test'). I've also uploaded a few screenshots to MediaWiki[4].