Downtime notification isn't entirely a technical problem. If there is a policy of user notification of downtime on this box, do so via agreed-upon channels.
Regarding control characters, not with wall or write. util-linux README indicates this is intentional.
Version 1.32b (12-Mar-95):
Write/wall now strips control chars except BEL (\007). Again after
suggestion by Rik Faith.
wall usually work, specifically if you use a file argument.
figlet is another option for your ASCII art.