Hi, my suggestion is to allow an array of low cost deep space beacons to be created so that they can be spread across the boundaries of a system. I would want this to provide an alert (initially via the current comms/discussion mechanism but hey why not expand on that later to trigger emails or whatever).
I would position the beacons at various location on the boundary of my system where a none-visible straight line geofence could exist between beacons which could alert me to when others have crossed into or out of what I consider to be my system or territory.
Once in position you might consider adding alert messages to be setup the intruder alerting them that they have strayed into a system and guiding them to either ‘bugger off’ or contact the system owner to negootiate safe passage.
A later evolution of this might be to allow the geofence trigger to cause an automated defence response if the player is not online at the time e.g. cause the system defence fleet(s) to attack intruders to chase them across the border or kill them.
Just a thought