I don't think you can justify letting a player, QB or whoever else, get into a play as a potential tackler or blocker and then restrict who can hit him. If a QB does not want to be hit like every other player then do not get into the middle of the play. If he was on the other side of the field I could see it being illegal, but not when he was in the middle of the play.
As for a judging at game speed that a player is "on his way down anyway" that is not going to happen and shouldn't. You play until the whistle, having some players think he is "on the way down" and others still playing because they don't see it the same way will lead to more injuries.