sorry page redirect

As an alternate look into the HTTP profile, you can do fallback host/error codes & then attach a simple iRule to it if still needed.

LTM Concepts - LTM HTTP Profiles

there are a few issues with the iRule as-is

• In the description, you mentioned sending a redirect to the formatted page on the server if the formatted page can be reached, but in the example there is nothing verifying that the formatted page is available
• The HTTP_REQUEST event cannot check for response content - HTTP_RESPONSE is more fitting
• The syntax used to send the response is wrong

To address the first point - I would create a monitor to check the status of the server's sorry-page and apply it to a new pool that only contains the sorry-server:

ltm monitor http down_pool_mon {
    defaults-from http
    destination *:*
    interval 5
    timeout 16
    recv "200 OK"
    send "GET /_default.html HTTP/1.1\r\nHost: www.siterequest.com\r\nConnection: Close\r\n\r\n"
    time-until-up 0
    timeout 10
}

ltm pool down_pool {
    load-balancing-mode round-robin
    members {
        10.10.10.10:80 {
            address 10.10.10.10
            session monitor-enabled
            state up
        }
    }
    monitor down_pool_mon 
}

The associated iRule:

when HTTP_RESPONSE {
  check server response code - if 404, perform sorry-activity
  if { [HTTP::status] contains "404"} {
        check availability of formatted-sorry-page on sorry-server
        ...by checking number of active members in down_pool
        if there is at least 1 active member, send redirect to sorry-page
        if no members in down_pool, send HTTP response from F5
        if { [active_members down_pool > 0 } {
            HTTP::redirect "http://www.siterequest.com/_default.html"
        } else {
            HTTP::respond 503 noserver content "Site Temporarily UnavailableWe are sorry, but the site you are looking for is temporarily out of service If you feel you have reached this page in error, please try again."
        }
  }
}