We'll see what RR says as the play is being sent to him for his view. Yes, the 9-yard mark rule was met but you still have these players entering as substitutes and then one heads back to the sideline. 9-2-2-b would imply this is a foul for unfair acts.
As stated on the UM play... if this tactic is legal, then run it to your opponent's side of the field. Better yet, run it without substituting, but then it wouldn't be a hideout play or work very well.