Just a thought. Whatever technology is used, be sure that it works with (on) both visible and IR cameras. Most license plate cameras (such as toll boths) now have dual camera technology to get a more likely image capture.
Back in the olden days, the fuel tank was behind the license plate and you pushed the plate down to put fuel in. One of those with a servo would work here, but it would be a manual thing. Check behind you for cops, hit the switch and the plate faces down. Clear the known trap or toll booth, hit the switch again and the plate faces up.
And, you're running a ticket risk for an electrical glitch either way - either it blanks and you get a ticket for covered tag, or it doesn't blank and you get a red light ticket or toll charge.