The only thing I've ever seen from NJMVC is the following. No mention of vertical or horizontal mounting. Perhaps call/e-mail your local constabulary and ask.
Motorists will receive two license plates when initially registering
a passenger vehicle. For trailers, motorized bicycles and motorcycles,
motorists will receive one plate.
For passenger vehicles, attach one plate to the front bumper and
one to the rear of the vehicle, at least 12 inches from the ground,
but no more than 48 inches. The rear plate should be lit at night
and be visible from 50 feet. Using license plate holders that obscure
any lettering on the license plate is a violation that could result
in a fine of up to $200.
If a license plate is lost or stolen, report the incident to local police.
Retain a copy of the complaint. Replace lost or damaged license
plates within 24 hours of the incident at any motor vehicle agency
by turning in the damaged or remaining plate.
Out-of-use license plates should be immediately returned to MVC.
Return these plates to any motor vehicle agency or any inspection
facility. Ask for a receipt. Keep the receipt.
The use of forged or counterfeit license plates could result in a
$500 fine, 60 days imprisonment, and a driver license suspension
of up to six months.