Ball hits goal post
 

If a missed field goal / try attempt deflects from the goal post into the field of play, is the ball playable? Or is it dead?

Dead.

In all the major American & Canadian codes, once the kick hits any of the goal, it remains alive for one purpose only: to determine whether it scores a goal. Once it satisfies the conditions for scoring, or can no longer score, it's dead. In Canadian football, if the kick has touched the ground, it remains in play on hitting the goal structure, but is dead if it hits some part of the goal support (not the goal proper) on the fly.