It would be great to support a downtime landing page. This would go hand-in-hand with the downtime message that we can send to users in the system prior to the outage.
During an outage, there is no way for users to understand that the system is offline on purpose. A simple static page showing the downtime was planned would be helpful - not everyone reads their email or was in the system prior to the downtime notification.
Something simple such as this elegant 404 page: