The point of this would be to allow personal discussion with the author, but spam and driveby comments would be discouraged.
I'm sure this must've been tried somewhere?
We are currently experimenting with a solution to this on Hubski. We are allowing some blogs to mirror Hubski discussions of their content on their blog. Here's an example: http://www.theadvancedapes.com/theratchet/2013/1/28/evolutio... I've long seen many bloggers append a post with "discussion on HN" or "discussed on Reddit". We decided to skip the link, and to allow the thread to be embedded. It's not so much a commenting platform as a mirror to the Hubski discussion. So far it seems to be working well, and we have a cue of blogs waiting to be added.
I should mention that users can follow or ignore users, tags, and domains on Hubski, and what they see is a result of what they follow. I don't think this would work on HN or Reddit, as no one would want every post from a given blog to be submitted automatically.