[Rich1]

I do not see anything in the rules specifically profibiting a defender from breaking the plane of the free throw line after the release in order to block out (only restrictions on the shooter).

I don't think a rule is really needed because if the shooter holds his position then the defender won't be able to go far without displacing the shooter which I'll call a foul. Since most shooters set up within inches of the line I don't see how a defender could break the plane by much without displacing the shooter.

It will be a situational thing for me based on displacement or advantage/disadvantage and I'm sure anyone who gets called for such a foul will not be breaking the plane in the future.