Perhaps you gave them too much of the story. Simply ask for plates for an old home built utility trailer.
Even here in Indiana land of taxes they can be decent about home built trailers. I was asked to stop by the police station and get a police report that it was definatly home built and wasn't stolen. Then I was to take the police report to the BMV for plates.
No problem. I have 5 trailers I tow with regularly and one plate. The registration says red home built trailer so each trailer has a red frame
I think most every state give you 5 days to plate ANY vehicle otherwise we couldn't buy a new car here in the U.S.