The reason for my question is, as I explained above, neither the passenger on the Jumbo nor the passenger on the Pitts Special has any clue about the probabilities of the risks involved in the activity and, therefore, never truly “accepts” those risks.
It isn't really about "accepting" the risk, it is about the likelihood of an injury event occurring and whether a reasonable person might believe that the risk of injury existed.
Most Jumbo passengers know enough that they don't usually travel on third world airlines. Most passengers in light aircraft know that they are less reliable and their pilots generally less trained than is the case in the better airlines. I think most passengers in a 2 seat Pitts would understand they are significantly more vulnerable than travelling by major airline. They just don't want to be responsible if their momentary adrenalin kick turns to something more permanent.
Perhaps that little placard "Passengers fly at own risk" might be the ultimate truism?
Kaz