Ahhhh, gotcha. I was going to ask about presence. I don't need to now. 
Yeah, you'd need a custom app to do this. You could possibly modify @jwetzel1492's [RELEASE] HTTP Presence Sensor to suit your needs as it basically does what you need (queries a http endpoint and sets presence based upon that). You could use to ping google or something and then setup a RM rule to trigger on when the "presence" is away, meaning that the internet is down.
