You could create a group "all" which is private - hidden - no tags.
Every user is member of this group. So you cloud mention the group @all where all users are members.
When a user has a queued message waiting to be approved and receives a notification that it has been accepted, the notification appears as if it does not have a reference user (question mark): you can change this behavior and specify a specific user id ? (as I can do for welcome notifications)
Will be available in 1.18.4