I worked for a neurosurgeon who was a classmate of the doctor's who did JFK's surgery. He happens to be a history buff also and told me that when Jackie climbed on the back of the convertible she was retriving the back of the president's skull; that he could have been saved in surgery.
I believe it. She is not acting like someone who is seeking cover from a hail of bullets. For that, she would have been ducking down into the foot-well of the back seat. The way "Jackie" was crawling, seemingly with purpose over the back of the convertible, lends credibility to your post.