Hi, plugin author here.

It's not currently possible to post comments using the official API (or any of the unofficial APIs), but this probably wouldn't be difficult to implement using a standard POST request to the actual Hacker News web server. The harder part would be getting the interface for posting comments inside of Vim user friendly; it would probably be similar to how fugitive.vim lets you enter git commit messages.

I will definitely look into adding this to the plugin if enough interest is shown.