I would love something like this, in either format you mention. I've taken advantage of peoples comment-screening settings in the past to point out typos and such.

I think my preference to implement it would be to have something like a discussion page (does wikipedia calls it a talk page? I can't remember) as a user option? One of the big conceit/picky-reader problems seems to be determining whether people even want to hear it. I've also seen from both authors and readers some confusion about when/whether to engage in conversations.