I agree a VINTAGE casket may be a part of funeral history, but that's not what the PCS is about. Its about the cars. Its the Professional CAR Society, not the Professional Funeral History Society.
If a casket/coffin/Ziegler is period correct to the coach, then it enhances - just as a church truck in its compartment, mounted funeral flag, one man cot/gurney in a combination or ambulance, or any number of items that would have been used in an ambulance (suction cylinder, oxygen tanks, resuscitator, defibrillator, backboards, oxygen meters, folding cots, stair chairs, scoop stretchers, restraints, cervical collars, splints, traction devices, radio gear, extra linens, spit pans, urinals, sand bags, medical supplies, etc.) Also consider that vintage funeral parking sings are displayed with regularity. Or a beautiful handmade Victorian coffin in the back of a horse drawn. None of these items came with the coach John. The reason you were given I feel was pure hogwash.
All of these items are in fact one in the same yet for bizarre reasons are weighted differently. Acceptance of an ambulance being displayed decked out with accessories but not a hearse is transparent hypocrisy. Yes, I understand the public's perception of caskets/coffins fully. No, I don't drive around with one. (My teens National Casket Co rail box in Flint had a purpose and doesn't count! ) Doesn't mean there isn't a tactful way to have a stock period casket/coffin which remains closed in the back of a vehicle at a show where it isn't on 'display' front and center.
Take this analogy:
In the US, women have the right to choose what happens with their body after conception to a certain point where legally an embryo crosses into being considered a baby. Yet before that point, if you're responsible for the death of that woman, you'll take not one but two homicide charges.
This is precisely how I've always viewed PCS' casket rule. This duplicity simply should not exist in principle - without so much as taking/stating sides. Yet it does. Ambulance but not hearse. Redundantly got it.