I‘m aware it‘s been a while, but since this might be interesting to someone else landing on this page in a quest for answers:

There‘s XEP-0133, building on Ad-Hoc commands, that defines how to “Send Announcement to Online Users“: https://xmpp.org/extensions/xep-0133.html#announce

So basically you need to request a data form (https://xmpp.org/extensions/xep-0004.html) and send it back to your server. Typically you will need some admin privileges to do so.