I know about those brackets yet LEOs down here still say it's illegal even when the law clearly states you can use those brackets but it most be positioned at the right side of the bumper. By law you should not have these stickers displayed at any other part of the car other than on the ride side of the bumper.
LEOs will also tag you for front plates not being mounted on the front bumper of the car even if you have it displayed on your dashboard.
The whole front plate on dashboard thing is unsafe, IMHO. If you were to come to a sudden stop, regardless if you have it "pressed" or not, you do run the risk of the plate flying right at you and potentially injuring you or any passenger in the car.
BTW, good thread.
This is a nice way to see how different some states are compared to others.