I've had the following config tested :
rserver redirect sorry-server
serverfarm redirect sorry-farm
policy-map type loadbalance first-match test-policy
serverfarm farm1 backup sorry-farm
...and this is ok - but it will result in the client getting a redirected URL.
eg. If they visit www.test.com and that farm was down, they'll be redirected to sorry.test.com via the backup farm.
However, is it possible to 'silently' retrieve the sorry page from sorry.test.com but not redirect the client?
That is, if the main farm was down, is it possible for the ACE to retrieve the page at sorry.test.com and present it back to the client without the client seeing a change in URL (eg. they would still see www.test.com in their browser)?