Send announcement on non ad-hoc client

I have ejabberd up and running. Many of the users will be signing in from mobile clients, mostly android. I need them to be able to send messages that will go to all users on the system, announcements will work perfectly. I haven't found a good client for Android that supports ad-hoc commands, though. Is there a way to allow these users to generate announcements via a non ad-hoc client? Could I add the announcement JID, mydomain.com/announce/online, to their roster, or something like that? I've tried to do that on the CLI and the web admin, but it doesn't seem to work - I think because it is expecting a user as part of the JID.