Hey Mike,
If you're using the English latches as it sounds like, you're on the right track with a pullcord. Original cars used a leather thong running from the latch knob to a point on the door hinge.
I can't say whether or not the English latches are burst proof or not. Mine never popped open on their own and I never had occasion to kick my way out of the car.
They always felt pretty secure and made a solid, satisfying "chunk" when they closed.
Al