I work for a company which has its own web server they are due to have a complete power blackout over the weekend, meaning their servers will be down.
Does anyone know a way we could present a down status on a maintenance page or some kind of redirect so we can at least inform our users that the site is down for maintenance and not just missing/broken?
The best way is probably setting up a redirection to dummy server on your load balancer or border routers. If you have no such thing, then you can either try asking your provider about the options, or temporarily change the DNS record, provided that you reduce DNS cache timeouts before and after the change, so it takes effect immediately.