Possibly better way to do it is to make a content script (or a greasemonkey script - same thing?) that rewrites links in HN's page such that when you open them in a new tab, you can keep track of the corresponding comments link, and then provide a Page Action that shows up in the location bar, clicking on which, will open up the comments page for the link.
This won't work where you directly happen to go to a site that was also on HN, but will work for the 80% use-case where you are going to a site from the HN front page itself.
This won't work where you directly happen to go to a site that was also on HN, but will work for the 80% use-case where you are going to a site from the HN front page itself.