I've thought about this one before too as far as whether such a player commits IP by participating. The definition of substitute in 2-32-15 states "a team member entering the field to fill a player vacancy remains a substitute until he is on his team's side of the neutral zone." So he remains a substitute, not a player, as long as he remains on A's side of the NZ.
Combine this with 9-6-3 "No replaced player, substitute...shall hinder an opponent, touch the ball, influence the play, or otherwise participate.
So if, as in the OP, player never gets to his side of NZ but participates in the play, it is IP. If he does get to his side of NZ and then participates, it is just IS.
Rule 2-32-1 seems to provide for the definition of a "player" to be expanded to "when a substitute otherwise becomes a player" - as opposed to entering and notifying a teammate that he is replacing him.
Here's a post I made a few months ago...while not exactly on point, it MAY help.. or not. In 2-32-10, a non-player is defined as...."a replaced player who does not participate by touching the ball, hindering an opponent, or influencing the play"
Then in 2-32-15 "a substitute becomes a player when he communicates with a teammate or a game official, enters the huddle, is positioned in a formation or participates in a play.
Therefore, only through the wonder of rules-speak do we arrive at a non-player coming off the bench, for instance, becoming a player by getting illegally involved in the play.
As the result, the foul for Illegal Participation is not a non-player foul because the substitute formally known as a non-player has become a player!!!