Custom error page iRule with IP address filtering

You can use LTM policy to achieve requirement.

However iRule is also choice. Create data group

MyIPList

    when HTTP_REQUEST {
     if { [class match [IP::client_addr] equals MyIPList] } {
          pool alternate_pool
     } else {
          HTTP::redirect "https://[HTTP::host]/underconstruction.html"
        }
    }