Hey Phil, me again.
Normally, when entering or exiting a slider, I sit on the seat back and lower myself down using handles on the roll bar. Having head rests would make this more challenging.
In flight, your head never contacts the headrest. Your passenger may want to have a snooze, however, so maybe one on that side is useful. Headrests in cars are primarily for whiplash prevention in rear-end collisions.
Maybe you can design a removable one for your passenger.
One more caveat. Your MD-RA inspector will need you to demonstrate that you can reach your fire extinguisher in flight with your seatbelt secure. About the only place that the firex can be mounted in an A model is at the top of the right flap pushrod cover in the baggage compartment. Your then reach behind the pax seat in flight to access it. Headrests may complicate this.
Vern Little