@Roflcopter just pointed out https://judaism.stackexchange.com/ which has a big "Topic Challenge" banner at the top. We like the idea - how would this be possible to configure for Travel.SE?
Turns out to be a System Message. Case Closed.
As Mark Mayo pointed out, it is a System Message. Since we shouldn't use this, how about using an Event? Events are also displayed on the main site.