Why rule out remote "wifi" control of the ac? Understand the unauthorised access bit and wireless comms is not my expertise, but what about limiting the range of the transmission of the signals FROM the aircraft?
A ground station could reach the aircraft at long range, but if that remote station has to transmit back a randomly generated access code transmitted from the aircraft AND the range of the aircraft transmitter is only 50ft, then the station has to be near by - in other words another aircraft in close formation even if only to get initial remote control?????
Remember not a comms expert just a random browser!