Realistically, this will not often be called a foul simply because it will not be seen. The attention of the official is directed higher up. Even when a player is hit in the face, often the only way it is noticeable, is because the contact causes the affected player to flinch.

The tap is just that, a tap. Just enough so that the shooter notices but not enough to throw them off balance.

I have been told in several camps over the summer that this needs to be called and all too often it is not.

I disagree that a tap on the leg has the same effect as the jab to the stomach. There is a much stronger reflex to protect your "vitals" than your extremities. Depending on how focused the shooter is, he may not even feel the tap on the leg. A tap on the leg may distract you, but the same tap to the gut would distract you more.

As for whether it's seen, the further the official is from the play the more likely it is to be seen.