Kalle,
You and I are on the same page. A foul, by strict reading of the rules (perhaps?). But, if the receiver makes the catch an instant before, then the contact is legal, since he is now a ball carrier. Is a defender expected to wait until he is sure the opponent has become a ball carrier to make this low block? We see this type of contact frequently, and no one has suggested that this should be a foul.
We MAY be able to hang our hat on the last line of 9-1-6-b-3: "This prohibition ends when a legal forward pass is no longer possible by rule." So, if all aspects of the pass are legal, and the ball is first touched beyond the spot of the pass, then a subsequent legal forward pass is no longer possible. That being the case, the prohibition against an otherwise legal low block against an eligible receiver beyond the NZ no longer exists, after that moment. There is some very close timing, in these events, but they would follow the language of the rule.