Hey, this is a project I've been working on. The homepage has a demo of what the sidebar looks like (which overlays to the right of your browser when you open the extension). Feedback/questions appreciated!
Nope, first I've heard of hypothesis! However I have seen a few things like it/Epiverse (Genius is doing something very similar to hypothesis). Apparently during the conception of the internet web annotations were planned as a feature. I think we're still a way off web annotations (very ambitious), but a good bridge is a comments system, which I think we are ready for.
Thanks for the suggestion! I'll look into it. I am wary of asking permission from host sites because then it's not necessarily user controlled (plus there wouldn't be a huge difference between us and something like Disqus). There are potentially ways I could integrate things like host verification for comments though.
I hope this feature request would be useful to you.