Replace HTTP status response in returned URI.

This is actually a pretty reasonable approach Nathan. You'd necessarily want a pre-crafted HTML page for each possible 50x status message, so listing them in a data group is a good idea. Minor typo though:

when HTTP_RESPONSE {
    if { [HTTP::status] starts_with 5 } {
        if { [class match [HTTP::status] equals 5xx_codes] } {
            set redirect [class match -value [HTTP::status] equals 5xx_codes]
            HTTP::redirect "http://serveraddress.com${redirect}"
        } else {
             no crafted HTML for this error message - better to just drop then
            reject
        }
    }
}

Nothing major, but the data group would have returned, for example, "/error_500.html", which would have made the redirect URL look like this (notice the double forward slash):

http://serveraddress.com//error_500.html

You could also save yourself the potential hassle of a second server to host these error messages by hosting them locally on the BIG-IP and using iFiles to retrieve them.