...I suspect that the body type limitation is intended to prevent someone from putting together truly a frankenstein car.
To an extent, you're correct.
The "body type" was an addition after the original rules were written; the original rule stated something like ud/bd allowed within same make/model/engine size car. However, I don't recall that station wagons were specifically excluded at that time, and it soon became apparent that manufacturers - especially 'Merican ones - equipped their wagons with heavier-duty equipment, including heavier-duty engines, intakes, suspensions, and brakes. Remember, "back then" wagons were really nothing more than light-duty pickup trucks underneath, not these fruity-car unibodies
...so, it was some time later that the "body style" restriction was added.
Today, that is usually no longer the case. If you buy a Widget XR32 hatchback, it typically has the same equipment as the Widget XR32 "avant", and/or it has equipment differences that are irrelevant within the context of the IT regs. Ergo, within the context of this history, allowing (as an example) a Ford Escort hatchback to run the manual rack from the Ford Escort sedan, both of which are on the same IT spec line and certainly given both are specifically listed on that same line, is well within the original spirit of the rules. At that point it becomes the responsibility of the ITAC/CRB to determine significant differences in equipment due to body styles, should there be any, and place them on a separate line if necessary.
I can see both sides of the argument, now that we've attempted to deeply intorturate the words. I won't take a set on who may be correct, but it has always been my expectation/assumption that as long as the cars are on the same line, equipment can be freely switched. If this is not the case, I can easily think of several cars now that would be illegal (think of cars now that are running equipment combinations that the manufacturer never offered)...
Should this be taken up by the CRB, I predict they will freely allow equipment substitutions back and forth within the same spec line, and if necessary task the ITAC to determine which cars should be moved to a discrete spec line (the proper way to do it). - GA