I guess if your DOT has nothing better to do than check trailer tire codes on personal vehicles, then I wouldn't invest in it either. Having driven tractor trailer in Ohio from 2004 through 2012, I've never been checked on the semi I was driving, nor any of my personal vehicles, YMMV..........
I know from my experiences, I'd much rather have the 19.5 tires on any car trailer, over the 16 inch ST tires.
I have seen equipment trailers that were equipped with 19.5 tires from the manufacturer, so there must be some workaround when selling them to the public. Boar has a toll free number, so you can check into the legality before you plunk down your money.