There is no way to do this through the rules engine. What can be done is set up a custom watchdog report that refreshed every half hour that only sends when a device is not communicating.
This would involve using some creative excel logic to account for our device heartbeats (30 mins interval, and then 23 hour intervals)
Alternatively, you can develop an application that does this through the SDK. A link to our SDK webpage, my.geotab.com/sdk where you can read what fields can be done through the sdk.
Kevin de Sousa