Before the NCAA adopted their current substitution practices, I had a team attempt this very type of tactic. To the coach's credit, he told us they would try this, and I told him that his player had to get within the 9 yard marks. So, I/we were aware they would/might try this. So, following a down, as the teams were huddling, I saw this player kinda 'ease' onto the field, and knew this was it. Oh, I'm confused. What down is it? I sound my whistle, stop the clock, and move to confer with wing official on the side of the field where this 'hideout' player was standing. "Steve! What down is it? I have second down. What do you have? Oh. Second down. OK. Good. Thank you." Guess what. In the mean time, Team B sees this hidin' out dude, and adjusts to get him covered. They run the play. Incomplete pass. 3rd down. We moved on. They didn't try that tactic again.