I watched a visiting shihan at our summer camp deal with this kind of attack, and comment on it. I think someone already made this or a similar point, but, basically, one needs to control the arm before it is taken back. In a properly executed reverse punch, or any strike for that matter, the arm is not tightened until the moment of anticipated impact, at which time it is then retracted/withdrawn. Therefore, you can effectively get off line and grasp, block or parry the arm if you time it right, prior to the arm whipping back. Unfortunately, it can take years until one developes the coordination, speed and timing necessary to be able to execute such a defense, but it can be done.
By the way, I would NEVER go into a seiza position in front of any experienced martial artist who was attacking me.