I'm not sure exactly when it was added, but in 9.4.7 (and possibly earlier) you can also configure the fallback host to be used for specific HTTP status codes like a 50x:
Fallback on Error Codes
Type the HTTP error codes from server responses that should trigger a redirection to the fallback host. Typical error codes are 500, 501, and 502. If you are specifying more than one code, separate the codes with a blank space, such as 500 501 502. You can also specify a range of error codes, as in this example: 505-515.
Aaron
Nimbostratus
