what you can try to do - not test so can't confirm this is a solution.
You can add the redirect services under a content rule as a normal service - not sorry server.
CSS first tries to use regular service before redirect services.
So it should use the local servers as long as they are actives.
Use the sorryserver to point to a webserver somewhere else in your network that displays a message that everything is temporarily down.
Like this you guarantee that the CSS will always answer the DNS request and when everything is down, people are shown a page with a correct error message.
Regards,
Gilles.