I've never been in a plane that made a hammerhead, but a someone who was says they just don't happen in AH as they should. As far as I understand it, a hammerhead is flipping around a yaw axis at the top of a vertical, with the axis somewhere between the nose and tail of the airplane.
I can do it in the game, but it's pretty slow compared to almost every hammerhead I've seen on video, although those real planes are probably a lot lighter than the fighters in AH.
The most efficient hammerhead in AH, in my experience, is one where the stall horn never sounds.