"I recall being told by someone that once the gear has cleared the ground that a signal is sent to each door that arms a latch in the door preventing the manual arm from being used and that the cabin crew can override this and or the crew up at the pointy end of things.
Not on the several Boeings I've flown, can't comment about the other brands.
As you say, in a pressurised aeroplane the differential/internal pressure is about 8psi, it's actually around 9psi, that's just over 1/2 a ton per square foot. A B747 main deck door is approximately 6 feet by 3 feet so there is roughly 9 tons of force holdng the door shut when the aeroplane is pressurised. Neither you nor Superman is going to move it inflight unless the aeroplane is depressurised first.
Once the gear leaves the ground a signal is sent to the pressurisation system to pressurise the aeroplane, if it was pressurised on the ground it would be impossible to carry out a ground evacuation if necessary. When the aeroplane lands another signal is sent to the pressurisation system to de-pressurise it so that the doors can be opened.
There is no extra latching system for the doors and therefore no override function.
Regards,
BH.