Why not create another host with the same domain but a different RSS feed. Does that not work? There’s also the option for Key to pull discourse username from feed. That may be helpful if you can alter the feed itself to include the username.
That said, I don’t see a plugin-outlet on the embedding page or a widget, which means there’s not necessarily an easy way to tie into that section without monkey-patching. And that’s something that is sometimes necessary but rarely recommended and far from ideal. I would try to find a different way to do what you want.
Maybe try an RSS importer and then embed on your site using the existing topic method (see Embedding Discourse Comments via Javascript and scroll down to “Linking to existing topics”).