Gomrath, thanks. Good one.
As far as I'm concerned this settles the issue: You cannot do flight training on a B-1 or B-2 visa, if that flight training is substantial or leads to a potential vocation, and particularly not if the primary purpose of your visit is to do the flight training. And that implies that you also cannot do these kinds of flight training under the VWP.
Obviously a PPL course is considered substantial and may to a potential vocation as it's the stepping stone in a modular ATPL. But whether a "fun" rating such as taildragger, aerobatics or seaplane would also be considered substantial, or leading to a potential vocation, I would not be able to say.
Anyway, the subject of this thread is about an intensive PPL and I think that issue has now been settled.