It's an interesting point...and F1 and Rally drivers use them, not necessarily for impacts but for the high and never-ending G forces during braking, although strictly speaking, HANS devices are not the same thing as simple neck braces.
Itchiboots wears a REV'IT Segur neck brace both on and off road, £170.
Doesn't seem to affect her head movement judging by the swivelling chin-camera shots.
It's a cheap enough safety gadget but there doesn't seem to be much data on the need for one, ie how many upper back/neck injuries are caused in bike excursions? Most of the headlines are about deaths or lower limb injuries.