I think it would be very simple. If an unscrupulous coach violates the rule, the word would get out fast because everyone knows everyone in the soccer community. Someone would quickly drop a dime on the offending school/coach I think. From there it would not be difficult to prove the violation. All that would be needed is a school roster, a few phone numbers, and time to interview the coach and players. Circumventing the investigation would require collusion among the coach, the players, the school, and the club team. Not impossible, but highly unlikely to succeed, in my opinion.
Having said this, I don't like the proposed rule [isn't it interesting that no one is in favor of the rule, yet it's still going to become a rule?]. And I agree with previous post that it would create a new administrative, regulatory burden on the coach and school. Useless work to comply with a useless rule.