I feel the pain, I have two trailers here without license plates. One, the owner never changed or registered the title. I have that title w/his signature in hand. The other had some composing issues by myself that the State rejected the title transfer. In both cases the MN DMV wanted dated signatures from the previous owner as to the veracity of the title transfer.
FWIW, I am going to skirt that black hole of bureaucracy and register both of those trailers and an additional utility trailer as "Home made". I found a clerk in an exurban registrar's office that is amenable to practical reason. Heck, I want to pay the State and help keep the roads up but their actions belie their purpose.