I figured there was probably a simpler way, so I threw together this userscript to merge the two.
1. Clicking a link from Hacker News opens the article with a side panel containing the discussion. It doesn't require your credentials, is resizable, and is easy to tweak if you want to customize it.
2. If you land on an article that has previously been shared on HN, the script finds the existing discussion and adds a button in the top-right to open the panel.
Feedback welcome.