On a thread started earlier, there was a question about legal guarding position with one foot out of bounds. It occurs to me that if this guard gets a hand on the ball, then the ball has gone OOB and A gets a throw-in. Would that be good strategy for the B team to practice? It would be a great way to stop a drive without an intentional foul. IF the ref would call it correctly. Or is it illegal? Or what?

There's nothing illegal about B setting up with one foot on the sideline. Is he OOB? Sure he is. But you know what? Players step OOB all the time.

Yes, you're play would be an OOB violation on B, A's ball for a throw-in.

So, if A is barrelling down the sideline with the ball, and B sets up and touches the ball but doesn't foul, or interrupt the drive, how many refs would actually call the OOB and stop the drive? Should they call it? Of course, but would they? How many would see it but not call it? How many would not notice?

I would go with not notice. If you're reffing the defense this should be easy, but we know that does not always happen.