The Saints Superiore can have a License Plate "VROOOM"
The Saints La Fuerza can have the License Plate: "DR. ALPHA" or "SNT ROW"
Unconfirmed: The Shard can have a License Plate "SHARD"
Unconfirmed: The Bezier can have a License Plate "BEZIER"
Unconfirmed: The Wellington can have a License Plate "WLNGTN"
The license plate of the Zircon does not have a frame.
The license plate of the unused "Arms Dealer" variant Phoenix is "SHRPNL"
The license plate of the unused "Shaundi" variant Topher is "SHAUND"
The Komodo's internal license plate is set as "onllllyyyy - License Plate - only option dood", but doesn't have a custom license plate.
The Buffalo with the Eagle decals by default has 1% chance of having a special "EAGLE 1" license plate. Even if you find a Eagle Decal Buffalo, there is a 99% chance it will have a random license plate, which makes a Buffalo with the "EAGLE 1" license plate one of the rarest in the game.
I believe that is everything, please correct me if I'm wrong.