I was at a lecture by a racing Doctor who treated Zanardi after his accident. He also worked on the development of HANS. I understand that HANS is a device which limits forward rotation of the head due to the seating position of the driver i.e almost supine (lying down). I don't think it would be very effective in a helicopter accident which would be characterised by excessive vertical deceleration.
In the case of excessive horizontal deceleration, you end up with a face full of cyclic or instrument console.
Rather than HANS, how about a full face helmet?