No, there is no such feature.
You can try sending an XMPP message to all the MUCs (chatrooms, which are used for the conferences) and it should appear in the chat of all the conferences. Not the same as the notice in config, but a chat message.
It’s just an idea, I haven’t checked this, so please have in mind that it may be more complicated or it may not work at all. I’m not sure from which user to send the message and how exactly to format it, etc. Prosody is extensible, so you can add/customize a plugin for this.
There was a XEP or a special feature in XMPP that allows you to send server maintenance messages to all the users of the server - I don’t remember how was it called and a very quick google search didn’t help me, but if you find this, you can try it too.