Reached through the plane and hit the...?

[BktBallRef]

Yes, he can reach through the plane and bat the ball after the throw-in has been released.

7-6-4
The opponent(s) of the thrower shall not have any part of his/her person through the inbounds side of the throw-in boundary plane until the ball has been released on a throw-in pass.

9-2 Penalty 3
If an opponent(s) of the thrower reaches through the throw-in boundary-line plane and touches or dislodges the ball while in possession of the thrower or being passed to a teammate outside the boundary line (as in 7-5-7), a technical foul shall be charged to the offender.