The following information was provided by Mark K. on 15th August, 2008:
Just presenting info, use or disregard at users discretion. (transcribed by me so errors are possible)
From the H.F. No. 3800, 4th Engrossment - 2007-2008th Legislative Session from Section 43, Subdivision 3.
[note that this has been stuffed way down in the sections no one bothers to read closely]
"If the vehicle is a motorcycle, motor scooter, motorized bicycle, motorcycle sidecar, trailer registered at greater than 3,000 pounds gross vehicle weight (GVW), semitrailer, or vehicle displaying a dealer plate, then one license plate must be displayed horizontally with the identifying numbers and letters facing outward from the vehicle and must be mounted in the upright position on the rear of the vehicle."
"It is unlawful to cover any assigned letters and numbers or the name of the state of origin of a license plate with any material whatever, including any clear or colorless material that affects the plate's visibility or reflectivity"
2008.08.15 hn