This is where one would find the requirements for getting equipment certified for the airframe. Whether it is Gulfstream, Bombardier, or Boeing, the manufacturer, or the company building an aircraft interior for individual customers, would know how to do the certification process, I would certainly think. I have not been in the aircraft maintenance business since 1981, but my recollection was every Gulfstream had a custom built interior, so they would know where to go. Not sure Boeing even sells aircraft one at a time, or at least one with an interior installed. All that said sure makes sense that if one could buy their own 757 the expense of a theater system would be pocket change; and getting the equipment approved would be the biggest problem, but if one started when they ordered the aircraft they could probably get approval by the time they were installing the interior. Sounds like a fun question. Has anyone e-mailed (insert your favorite billionaire)?
TITLE 14--AERONAUTICS AND SPACE,
CHAPTER I--FEDERAL AVIATION ADMINISTRATION, DEPARTMENT OF TRANSPORTATION,
PART 21_CERTIFICATION PROCEDURES FOR PRODUCTS AND PARTS--Table of
Contents, Special Federal Aviation Regulation No. 88