Do you think people are going to wait for someone to submit each post to HN? I think this will encourage writers to submit every single story they write to HN, just to get a comments page.
I think if I were to implement this on my own page, I would probably have two tabs that expand if clicked. Something like this:
Show [your site's name] comments | Show HN comments
And then for the HN comment tab, you would use the OP's awesome submission. Thanks, btw!