I thought this was quite an interesting idea, actually. It could probably be done within HTML files using a new conditional tag, or something like that. Or it could be very simple to get included as part of the ePub specification. Of course, people could write a book viewer that would ignore the tag, just as people write ad blockers for browsers now. The trick would be to make the reminder notice
just annoying enough to maximize the number of people who would feel like paying for the book, and
convenient enough (e.g. an embedded link to a paypal site or something like that) so that people could make the payment quickly and easily and get on with life.
Naturally, no system is 100% perfect, and
any DRM system can be circumvented. But if a system only provides a reminder that blinks at you for 10 seconds when you open the book until you pay for it, probably no one would bother circumventing it, and probably many people would pay for it. How many? Not everyone, but
possibly more people would pay for the book if it were very widely distributed using this scheme, than would download it with an upfront payment to the author's site or a bookstore. And the costs of distribution would be lifted from the author.
As I said, I think it's an interesting idea. How would we actually test its value, rather than arguing about it? Could we find a set of books similar enough that we could try releasing them under different models and see which one generates the most revenue for the author? I think that would make the best test.
Though I suppose that depends on whether an author feels it's best to have the book "paid for" by the highest percentage of readers, or by the largest raw number of people. Personally, I'd go for highest raw number or highest revenue, even if it meant a lower percentage of readers actually paid for my work. But I'm more a pragmatist than an idealist.