BOAC, you are correct in saying that the airplane is slightly pressurized on the ground with AUTO mode selected on the pressurization control panel and all doors closed. In fact cabin altitude will be 70 feet maximum below field elevation at sea level which equates to approx. less than 1.25 psi. I have never tried it but I'm fairly certain that cabin doors can be opened at that differential. By the way, there are no DV windows on the 747.