The suggested commenting system was sorta implemented by Microsoft a long time ago (IE5?). Internet Explorer had a plugin called "Discussions" or "Discuss" or something to that effect. It let you have comment threads on any webpage, without involving the actual site.

I'm not sure if there was a public service, or it was more of an Intranet type thing. I do recall seeing the Discuss button in IE much later then IE5, and a quick search shows plenty of people asking what it is, so it mustn't have been too obscure a feature.