There are a lot of places where it's illegal to mount your plate sideways.
If you're going cafe, I would see if there's a way to mount it up against the back of your seat or use it as a sort of makeshift rear fender (mounting it below the seat, down and out 45 deg). On the stock setup it's directly below the light so that a hole cut in the bottom of the brake light illuminates the plate. Can you give that a go?