One interpreter got back to me and said...
Bruce,
Something crazy always happens in baseball! Yes, the pitch still meets the definition of a pitch; however, your play would not be an HBP. Good no call! I base my answer on the fact that the batter had his opportunity to hit the baseball, and that ended as soon as the ball/pitch got by the catcher and ultimately struck the backstop. Suppose you had a situation where a pitched ball careened from the umpire and then somehow hit the batter? Again, no HBP, but the definition of pitch is still in effect until, by rule, it is no longer a pitch. I believe that you should make your call/ruling by citing Rule #10, in which (we) can make decisions even though they are not clearly delineated in the rules book. Perhaps, this would be a Situation which should be added to the NFHS Case Book.