That would be an ineligible downfield. In order to legally "dump" the ball, the QB must throw the ball beyond the LOS, which means all the usual downfield pass restrictions are in effect.
One could potentially make the argument that it's OPI for an A player blocking downfield during a legal forward pass. By rule, I would agree that it is. By philosophy, since the ball was dumped into the third row, the downfield blocking didn't hinder the B player from making a bona fide attempt at catching the pass. And before anybody jumps on me that 7-5-11c only applies to B players, this is the same philosophical reasoning behind not calling holding on the right side of the line when the run goes to the left.
As for whose call it is, this is the odd situation where the BJ could make that call for ineligible downfield -- I've personally done it before when I realized that A55 was standing next to me as the pass was coming in. As a BJ, I wouldn't go fish in the U's pond and claim A55 was blocking 5 steps too far downfield, but if he's in the BJ's pond 20 yards downfield he can absolutely make that call himself. If you go with OPI, then BJ has that call all game long.