I happen to know someone who was on Lincolns, Washingtons etc. Then on UK civilian flights over Russian territory ( known as the ' Getting Lost Squadron ' ) he'd be invited to the cockpit, instead he'd descend to the bowels of the aircraft with a camera.
I don't know how they got around pressurisation, presumably a mask; I take it modern Nav-aids make the excuse even less plausible, and such practices may have led to the tragic downing of the Korean 747, whether it was involved or not.