Maintenance page per status code received

So your irule will work as-is for when there are no servers available. Now you need to look at the responses when there ARE servers and intercept/replace that response. So just add this to the bottom of your iRule.

when HTTP_RESPONSE {
  switch [HTTP::status] {
    "3*" {
      HTTP::respond 200 content "Your 3xx content here"
    }
    "4*" {
      HTTP::respond 200 content "Your 4xx content here"
    }
    "5*" {
      HTTP::respond 200 contetn "Your 5xx content here"
    }
  }
}

This is very basic and there could certainly be some further error checking, logging, and handling done if needed. This will give you a start though.

Joe