Not sure of the legality but Texas LEOs (since I started riding in 1998) get very annoyed if the plate, inspection sticker, and plate light aren't obvious from the back of the bike. If you try to hide any of these you will probably be on a first name basis with all the local popos.
If you put it on the forks you will spend some time explaining it. When I moved back to TX from VA (where they go on the forks) I got pulled over a lot. The plate tag holders are $5.99 and bolt to your plate. That is the easiest way to stay off the side of the road.