John, who would have thought being a fat b@stard would be a health benefit....
The Leatt Brace is the best thing I've seen so far - prevents cervical movement in 360 degrees very well without restricting normal head movement - however;
Helicopter helmets typically go no lower than required to get the earcup over the ears. Motorsport helmets cover the entire skull, jawline and then some. In a downward or forward accident, the head will naturally tend to travel downward and forwards. The likelihood of the jaw being the item that stops travel of the head is therefore high.
Gentex offer a maxillofacial shield for the HGU-56P helmet, but I doubt it's designed to accept such a load.
I wonder (and I'm shuddering at the cost) if a specific brace compatible helicopter helmet may be required, since we probably won't get the features we look for in aviating, not to mention looking a bit silly wearing motorcross helmets.... (not that it's ever stopped me before).