Pondering on the boat running away problem:
I notice my phone has both Bluetooth and WiFi
Becoming tired of pulling wires and repairing corroded sockets and plugs I am exploring WiFi
and there are some interesting things coming onto the market and as has happened with comms and computers
I foresee the day, not far in the future, when everything which may have a comms capability or requirement will come with WiFi.
It would appear that what we need is your system to tell the phone that someone's signal has stopped, and, an App that has the phone call the WiFi capable nav or autopilot
via it's WiFi to have it respond.
Before we go nuts re how the system might respond and all the reasons it would not work, consider the alternatives. A boat set up to sail which luffs up or goes into a heave-too status or under power de-throttles/shuts-down/circles-at-idle, or one which sails
or steams over the horizon at 5-7 knots.