not sure it would help much in fact you'd need to be careful that it's not going to change the angle of your neck too much but could be helpful to cushion the back of your head against flying debris...
An inflight magazine will do a good job against sharp objects.
The average seat cushion does little to absorb energy in a serious accident.
In fact I'm told by an aviation engineer that a soft cushion isolates the body from the initial deceleration when the aircraft frame distorts at the initial impact. This is a bad idea.
It is only for a fraction of a second but it counts. So instead of the passengers body continues to travel at near the original speed. What happens next is the initial millisecond deceleration is over but the body is travelling faster than the airframe so then hits the seat bottom with greater impact than if there were no cushion.
His rather blunt advice is unless you have a purpose designed "hard", energy absorbing seat it is better to remove the cushion, put it on your head and bolt your spine to the airframe.
