My guess - and its only a guess and moot at this point - is the trailer would not qualify for antique plates. Here in Ohio, we are limited by the number of miles an antique vehicle may drive in a given year. How would you document that for a trailer? While there might be a loophole that allows trailers to have antique plates, I doubt the bureaucracy would allow it - after all, regardless of how old the trailer is, it could be pulled behind one of several motor vehicles. I've seen trailers that look like they made it through the ice age...and have never seen any plates other than standard trailer license tags.