Whats wrong with the server chat? For a repeated message you could use the Advertising system. Add the remind message, and some hours before you take down the server, you enable it to write this information every 30 minutes (or something like this) to the server chat. You can enable the advertising system by bot chat commands.
If you already use the advertising system, just create a second advertising messages file (which includes the server down remind message) and change this file with bot chat commands.