Serve maintenance page if file exists

Here's what Cory's description might look like:

ltm monitor http http_file_exists_test {
    defaults-from http
    destination *:*
    interval 1
    recv 404\\sNot\\sFound
    recv-disable 200\\sOK
    send "GET /foo.php HTTP/1.1\\r\\nHost: server\\r\\n\\r\\n"
    time-until-up 0
    timeout 16
}

It's essentially the reverse of a normal monitor. Depending on how the web server responds to a file not found error (ie. 404 status code), the receive string would be the 404 response (doesn't exist so monitor is up), and receive disable string is the 200 OK (exists so monitor is down).