[Jim S]

Illini, Don'r get caught up with using pass interference rules behind the LOS. They don't apply. The prohibition of contacting a pass receiver who is no longer a blocker applies only to pass interfertence and must be beyond the LOS.
There is no prohibition on contacting a potential receiver behind the LOS as long as it is not an illegal block (a block that does not meet the same requirements as any other block) or is not a personal foul.
Other than that a defender may block a potential receiver behind the LOS. Even with the ball in the air. The reason you can't find a rule permitting this is that there is not one to find. But remember, the rules don't list what is legal to do (in most cases) but rather what is illegal to do. If there is not a rule prohibiting an action, that action is legal.