In response to CalhounLJ's post, IMO we continue to officiate and if there is a player that is continuing to engage in illegal activity this needs to be penalized to the point up to including a score if need be. If A58 begins to hold at the line of scrimmage and practically tackles B80 while chasing him into A's backfield all the while chasing the runner the QB in this situation and just prior tackling this runner or QB A58 finally gets B80 to the ground during which the QB throws an incomplete pass, what you are saying is the spot is the line of scrimmage. I think not. Or if A 40's helmet comes off while blocking and then Team A loses possession of the ball and B picks it up and is running for a touchdown and A40 tackles him B would simply have to decline the penalty where he lost his helmet to keep the ball. The spirit and the letter of this rule is where he participates and to me this includes when he "continues" to participate.
I agree with that philosophy, and if that occurred in this situation, I would agree that there could be and should be an untimed down. I just don't see it in the original post. There is nothing that indicates the situation you described happened. I will add that simply being on the field is not grounds for a new flag.
This casebook play supports your position:
9.6.4 SITUATION E:
With third down and 12 yards to go from A's 45-yard line, quarterback A1 throws a pass to A2 at B's 45-yard line. Teammate A12 moves to get a better view of the play and enters the field of play at B's 40-yard line. During the run after the reception,
(a) A2 collides with A12 and is thereafter tackled by B1; or
(b) B1 slows to avoid A12 as A1 runs for a touchdown.
RULING: Illegal participation in (a) and (b). The penalty of 15 yards is enforced using the "all-but-one" principle. Illegal participation occurs since substitute A12 hindered a teammate or an opponent and participated in the play. The spot of the foul is where the participation occurred, not where A12 entered the field. (2-30, 9-6-4a)