The thing with the old 707 story was that Tex Johnston managed to keep a constant 1
g throughout the manoeuvre. He knew what he was doing, which is why he was Boeing's chief test pilot.
In theory it's entirely possible with any aeroplane, as long as you keep at a constant 1
g. Of course, it's not exactly legal with most aeroplanes, and for good reason.
There was an accident a while back in America when a pilot with no aerobatic experience attempted to barrel roll a Beech Baron,
a la Tex in the 707 prototype. The wings fell off and what was left of the Baron sans aerofoils fell to Earth rather quickly, along with its unfortunate pilot. Oops
Smithy