I don't have a screenshot on hand, but I know there is some kind of "site unavailable" page with Jive branding. I don't know if it can be customized - I'm going to guess that it can't for Jive-x (cloud) customers, but probably could be for on prem.
Someone in Jive Support should be able to provide more information about outage messaging.
This is helpful -- I will double-check with them as well! Was more that I just wanted to make sure it was actionable/informative enough (i.e. "We're experiencing difficulties right now, and are working to rectify the situation....please call support channels for immediate questions with the Forums being down...." etc. etc.). Will confirm with the team. Thanks so much!