That's a good idea. Rapid shedding of altitude should automatically pop the lock. Say, 20,000 feet descent in under 5 minutes. That is very clearly not normal behavior of a large plane, which takes 30 minutes to reach cruising altitude of 30K+ feet, usually in small increments of 1000 feet every 2-3 minutes after the initial rise.
If the plane really is crashing due to catastrophe, popping the lock on the cabin door won't make a difference. If it's crashing because the pilot saw a vision of Jesus who told him to kill everyone on board, it might save the plane.