I've been working towards building out functionality close to what you're describing. As a first feature, I added annotations where the script looks for HN comments quoting parts of the article and adds a highlighter effect. It lets you instantly see what parts of the text people are actually interested in. It's still a rough beta, but I'm actively working toward that exact vision.

I started working towards this goal in the latest release: you can now add Reddit as a source, so now when you visit a URL it will check across all subreddits where it's been posted and give you a blended discussion made up from everyone's comments.