I have a line on the Miata seats with adjustable headrests (see Evan Amaya's for an example).
But. I'd need to sell my existing seats to make it financially viable. If interested, PM me and we can discuss.
Note, the seat backs have a slot in the middle just below the headrest that could be stiched back up. (I had originally planned for a Schroth 4-point harness that came together behind the shoulders and then had a single strap going to an inertia reel. The idea was to have the racing harness look with the ability to lean forward like a regular seatbelt.)
RobFicalora-EI.JPG
You better snatch those Miata M seats fast! They seem to be quite scarce.