There are LCD license plate frames which go opaque/transparent when powered, for example http://www.ghostplate.com/mechanics/ http://www.ghostplate.com/mechanic

There are LCD license plate frames which go opaque/transparent when powered, for example http://www.ghostplate.com/mechanics/ http://www.ghostplate.com/mechanics/ (first google result for "lcd license plate frame"). Of course they're Very Illegal to use while driving.