We are currently developing a school district's websites in Multisite. Is there any way for an administrator to set a network-wide notification (like a simple red bar at the top of the page) for every site on the network at once? I've looked around and found little on how to make a plugin or php post data to every page on a network. I can only assume this can be accomplished by some lines of php and/or some added functionality to the WP core but have no idea where to start.
Any ideas? I was thinking maybe a custom post type that you could schedule in one blog that automatically pushes to each of the blogs individually (that then just outputs some code), but I was having some difficulty wrapping my head around the problem.
All it would need to do is present static data like "Schools Closed Today:
<?php echo date ?> on a date that could be set (or, even an "on/off switch")