Check the local law on plate mounts then go from there.
real shame that vertical mounting is illegal in many states
I'm not expert on this, but is was AMA Mag that reported in their Sept '08 issue that Florida passed a bill (Bill 137) that requires all motorcycles in Florida to have horizontal license plates, and be incapable of being flipped up or adjusted.
Same florida bill mentioned requires that a motorcycle must have both wheels on the ground.
Check the local law on plate mounts then go from there.