Speaking as a "fatass GS worker" (no offense taken -- there's too much truth in that line

), please understand that NAVAIR does not prohibit any operational folks from flying any which way the want to -- NAVAIR is not in your operational command-chain. All we can do is refuse to grant technical approval of such a configuration. If the PIC or (more likely) the operational commander then refuses to let you-all fly with that gear, it is ultimately their call -- not NAVAIRs. Of course, the Commodore or Skipper or whomever will make his/her decision with NAVAIR's input in-mind.
As to the supplemental (i.e. carry-on / carry-off) seat-suchions that you-all are talking about, there is a very simple reason that NAVAIR has never approved them: Nobody had ever asked until recently!
You-all can ask for NAVAIR to evaluate pretty-much anything (not that NAVAIR will approve unsafe products, but you can always ask) via the NAVAIR Airworthiness Website at
https://airworthiness.navair.navy.mil/ This is a waaay underutilized resource and, when it all comes down to it, NAVAIR ultimately should work for you -- use this website to force NAVAIR to do so.
NAVAIR is evaluating the OA cushion-set and the Wings Unlimited butt-cushion and lumbar pad next week on the T-34 at PAX. One of your TRAWING JOs is submitting an Airworthiness Request to NAVAIR on Monday which will enable NAVAIR to do a cockpit fit-check, ergonomic assessment, FOD-eval, bail-out drills, etc. Whatever is necessary to evaluate and (hopefully) approve the cushions for purchase & use. NAVAIR will try to do the TH-57 next week as well, but has not yet confirmed acft availability.
*****edit with poster permission*****
Now I know that neither the T-34 nor the TH-57 has ejection seats (duh!), but this is just an example of the subtleties that go into acft design and human-integration (like the hand-held GPS that fired a Tomcat's front-seater after a trap).
That's why you-all (indirectly) pay NAVAIR to evaluate and assess this stuff -- so that you don't end-up with your head ground off on a wet runway in South Carolina, with the last thought going through your dying brain being "But, it was only a cush...