On the outside of a 747 classic while talking to the engineers in the cockpit after a pressurization check we couldn't open the main entry door even when the cockpit pressure was indicating zero. The butterfly handles wouldn't budge past a certain point, eventually the upper and lower gates opened with a burst of air rushing out.
As said above the negative relief valves are probably the best bet if you don't want to damage the aircraft by punching a hole in it.
Anything other than near zero diff pressure and you better use an axe or considerably larger if you're in a rush.