Hi there, @olon
Can you explain a little bit better on what you actually mean by broadcast channel sending messages? As you may know, every member of the Mattermost team is part of the Town Square by default. In this case, you can use @channel (for every users) or @here (for online users) depending on your preference to send messages to them. This was explained in an earlier post here:
Notification to ALL users on Mattermost
Since I am not entirely clear on your objective, I would prefer to get more information from your end first. Keep me posted.