N5528P:
I have heard that even deadheading crew members are not allowed to ride in the jumpseat...
and
Has anyone ever heard of a case where a visitor to the cockpit has tried to attack the flight crew?
No, I havn't heard of a visitor to the flight deck attacking the flight crew but there is a well documented case where a deadheading crew member sitting in a jump seat on a cargo flight in the US attacked the flight crew with a fire axe, seriously injuring both the operating crew. Fortunately, the operating crew was able to overpower the off-duty pilot in the jump seat and land the aircraft safely despite their serious injuries. I believe the captain of that flight never flew again.