As @wycats said here you can actually use {{render "comment" this}} multiple times. It only uses a singleton controller when you don’t give it a model.
You can also use that in the comment template, but this also brings a question. Why do you want to extract the author logic from the comment at all? If I imagine something like Discourse and it’s replies
I don’t think the author box is reusable, since it always looks a bit different in different parts of the site.
But maybe I’m missing something here? 