MN has a law that allows SOME military vehicles to be registered. It does NOT have anything to do with FMVSS, but is based on vehicle size and weight.
See subdivision 1(b):
(b) A decommissioned military vehicle that (1) was also manufactured and sold as a comparable civilian vehicle, and (2) has the same size dimensions and vehicle weight as the comparable civilian vehicle, is eligible for a certificate of title under this chapter.
This means that CUCV and older HMMWVs should be able to be titled and registered in the state. MN still has the letter up saying HMMWV's can't be registered, but as long as you can show the size and weight are the same as a civilian vehicle (Hummer H1 of any year), this should be enough proof. Just have to find the correct registrar to actually follow the rules.