That is a very good question. There are no AR's that cover this, and I can't find a bulletin play that covers this. The purpose of the rule is to protect a passer from injury. The defensive players can see that he is attempting to pass. The rule does not say that that the passer is to be "stationary," only in a passing posture. Even if he is moving backward, it is obvious he is not attempting advance, and he is attempting to pass. I'd say that is a passing posture. IMHO - which ain't official - I'd err on the side of safety, offer the passer this protection, and let someone else tell me I was wrong.
Robert