In the body shop we have done a lot of different types of tail gates. the one I like the best is to smooth everything from the outside and weld seat belt latch's in at the top. I put the female part on the inside of the bed and the male on the tail gate. You have to grid a little of the top male part to get it to roll in smooth. I like the GM latch's the best and just push the button and pull to you and it's open. Good luck and have fun.
If you wanted hidden hinges you would have to use a similar hinge as you would with a door. Of course it would have to be of the type where the door swings OUTSIDE of the adjacent panel. You could make these hinges pretty easy.