-possibility to add some custom temporary text when there is service outage
-option to add custom historic entries (for example when service is up again, user could write detailed info what went wrong, and this would be visible in timeline)
+1 on adding custom history entries. Best for post-mortem, or just to show how we handled a down time
Another possibly good option is to embed a Twitter widget, showing a company tweets stream, possibly tweeting about the down time. It's more lively, but it doesn't replace the custom text and the custom history entry.
The Twitter widget can be used for that, so it's probably fairly easy to do https://dev.twitter.com/web/embedded-timelines