The only feasible mechanic that I could possibly envision regarding this would be for the U to place the ball down at the LOS spot, and then to stand over the ball, until such time that the "shot clock" hits 29, and then to quickly get himself out of there before the ball is snapped. Ultimately meaning that we're going to need faster Umpires!
No U mechanic is necessary. Players can simply check the play clock to determine whether they can legally snap, and if they screw up, the B blows and throws.
The B's job gets a little harder - now he has to keep an eye on the play clock while also determining his key. Perhaps this call could go to the S or F, since they can find their key much more easily, almost instantaneously.
The hard part for the U is the rule we already have, when Team A substitutes and then the U *does* have to move and communicate that they're not allowed to snap.