I remember a very similar startup that was doing more or less the same thing which has been a very popular in HN and some other websites, I just could not recall the name. Anyone remembers?
Founder of Envolve here; yes, we did build a version of Envolve for HN called envo.lv
It was always intended to be a marketing gimmick. The critical mass required to sustain good conversation only lasted for the time the story was on the front page.
Getting a website owner to install a chat system, (rather than just having an overlay) combined with integrating with site credentials will make an on-site chat system significantly more viable.
This kind of thing is a classic chicken-and-egg problem. To actually have one take off, I wonder if there's some way to link it to Facebook, to make it viral?
Make it a Facebook app as well that interacts with comments you've left on pages you like? Or post your chat comments to Facebook? I don't know what it is, but I would love to see it happen.
edit: It could be envo.lv http://news.ycombinator.com/item?id=2756145