This actually just came up from one of your users yesterday here Motor shutdown on pwm input (parachute release).
ArduPilot can detect the condition (falling out of the sky) or take a manual RC input which will stop the motors and trigger an output to pop the chute.
What ArduPilot currently does not have is way for an external input other than the RC receiver to trigger the emergency stop. So if you have an external parachute device, that does the detection and chute popping, there is no way to tell ArduPilot to stop the motors right now.