GXPD - My E falls in to the same group. The tube inner dia for our models is .320" (140092-1) but at some point it changes .510". The center to center spacing of the tubes is 6.0" (140078-1). Oddly enough the parts catalog calls these tubes the headreast attach tubes, yet I don't see a headrest anyplace in the IPC? Anybody find that? The headrest is listed under optional equipment and I think the 6.0" between tubes remains the same up to an Acclaim. Only the tube dia changes and the design of the headrest itself so any of them ought to be interchangeable.
As far as the above comment 'whos going to care anyway' about the install, the optional equipment list does provide a weight/moment/arm for the headrests although I think I've seen a note that says the headrest weight is negligible for W&B. Not sure how weight can be 'negligible' though - it certainly wouldn't be on a newly certified aircraft! The other concern is burn requirements for interior components. The certification basis for our E models is the old CAR 3. (Newer models have some areas that meet newer FAR - see the TC 2A3 for more details to the cert req for your model.) So here's the relevant CAR 3:
§ 3.388 Fire precautions—(a) Cabin interiors. Only materials which are flashresistant shall be used. In compartments where smoking is to be permitted, the materials of the cabin lining, floors, upholstery, and furnishings shall be flame-resistant. Such compartments shall be equipped with an adequate number of selfcontained ash trays. All other compartments shall be placarded against smoking.
So the requirement would be flash resistant if you've got a placard for no smoking and flame resistant if you don't. You'd need to have material certs for the headrest that support this with burn data from an appropriate lab and an 8110-3 signed by an FAA DER. I do rules, here they are for whatever they're worth. You probably could obtain a later Mooney model headrest and install it in accordance with the Optional Equipment drawing legally with a simple logbook sign off. Ok, so everyone should go out to their cars and measure the spacing of the posts on their headrests - if it's 6", check the diameter of the tubes! The cars outside my house are 5 1/8" & 2 3/8".