[Juser] send "sticky" message to all clients?

[Kristian Rink]

Folks;

we're running XMPP internally for apprx. 50 people (Windows and Linux 
desktops, Android and iOS mobile devices). For certain purposes, we want 
to be able to send messages that will be noticed by each and every user 
regardless of device or client configuration. Requirements:

- Messages should pop up on screen, in front of all other windows, 
immediately.

- Message popups should stay "visible" unless they are manually closed 
by the user.

I see this mostly a matter of finding a good client, yet so far most 
clients we tried focussed on providing small, "decent" notifications 
(such as blinking icons which go unnoticed in our environment - or 
message sounds which don't work here at all as the systems don't have 
any audio devices connected).

So far, too, we use a conference chat for these purposes. Will this kind 
of notification work with a conference, or should we rather focus on 
sending out individual notifications to all users?

Any recommendations on that? How do you folks handle use cases like this?
TIA and all the best,
Kristian