Quote - The Billboard is pointed at the camera as before.  The dummy remains stationary, and anchors the billboard to the spot.  The dummy's job is simply to act as the axle on which the billboard rotates.
DPH

I loaded two objects, a plane and a cube.  I parented the plane to the cube and rotated the camera, and the plane still spins around.  As I expected, limits have no effect, as rotational values do not change when the object is "pointing at".

Next I added a sphere (dummy) and a cube.  I pointed the cube at the sphere and the sphere at the camera.  Rotating the camera allows me to pan around the cube as if the dummy doesn't exist.  

Had I not seen the billboard trees in the marketplace, I'd be doubting they can be made in Poser.