My wifes Nightster has one of those kits installed from factory, (2008 model year) apparently HD had to remove that option part way through 2008 because transport Canada refused to allow them entry as they didn't meet either licence plate requirements or lighting requirements... here is what I can find from the TSA...

Section 70 of the OPERATOR LICENSING AND VEHICLE
CONTROL REGULATION states:
Location of plates

70(1) The owner of a vehicle shall attach a licence plate securely as follows:

(a) if the vehicle is not a truck tractor or a motor cycle, to the rear of the vehicle so that the lower edge of the plate is not lower than the axle;

(b) if the vehicle is a truck tractor, to the front of the vehicle;

(c) if the vehicle is a motor cycle, on the rear mudguard or rear fender so that it is clearly visible.