I actually think that the "autograph-signing" is a hilarious example.
One of two things can happen.
First, a player can "agree" to go to an autograph signing that is organized by a third party, such as a memorabilia store. This type of arrangement will need to be approved in advance. Therefore, if the market rate for such an event is that the organizer pays the signer $5 for every autograph for which the organizer collects a fee, then it doesn't matter if a booster offers to pay the organizer an extra $100, the player would still be limited to the market rate. As in EVERY SITUATION, if someone really wants to slide the kid an extra $100, nothing will stop that process, and you certainly don't need to set up a sham autograph signing to give an athlete illegal money.
Second, a player could set up HIS OWN autograph signing (which would also need to be approved in advance). But now, let's say the kid wants to sign for $500 a pop. Yes, boosters could agree to pay, but the line for autographs would be short. And obvious. And it would likely be a violation of the NCAA's fair market value policies if someone did so. Again, if the entire goal is to pay a player illegally, there are easier, more direct, and far less sham-like ways to do so.