I just got a response from a friend who is a game official. He says that the NBA rule that "no player may cross the plane of the boundary once a designated thrower has been identified" means the same thing as the NCAA rule that states:
Art. 2. No player other than the thrower-in shall:
a. Perform the throw-in or be out of bounds after a designated-spot
throw-in begins.
Assuming he is correct (I consider him a good source) it means that in this situation, Nitwitzki was not allowed to go out of bounds. Which means the play was illegal. I looked at every clip of front court baseline inbounds plays I could find. I didn't see a single one where a player ran out of bounds.
If you assume that Nitwitzki was not supposed to run OOB, then Elson did exactly what he should have done. He picked up Nitwitzki after Bowen got screened off by Bass, and got between him and the inbounds passer. Bowen should have been able to stick with Bass, since Nitwitzki wasn't supposed to be popping up on the opposite side of the court.
I don't know if AJ knew the play was illegal, or even thought about it. He drew up a play that he thought would work. And it would have worked if Nitwitzki had made that shot. I just think the Spurs and the refs were both caught flat-footed, because that same play would have been legal on the other end of the floor, after a made field goal.
But don't have any doubts that was the way the play was drawn up. As soon as Bass made contact with Bowen, Nitwitzki headed straight OOB (and deep OOB) and headed straight for the point on the court where Ginobili was guarding the inbound pass. Howard stayed low to clear out anyone coming across the paint, which was Elson. Legal or not, the Mavs ran the play to perfection.
[As I was writing I got an e-mail from another friend. We may see that play discussed on a national sports show this evening or tomorrow. (I'll post if I find out for sure.) Should get some confirmation from a more "official" source if that happens.]