What you really want is threaded comments. This is very simple to add to general comments. It does not appear you want to actually make the comments available from the forums UI. The comments really only make sense in context of what they are commenting on.
Since general-comments are built on the content repository is to very easy to set the parent_id to a another comment.
You could base this on contrib/packages/general-comments, which does not use acs-messaging, to simplify the code, and already supports the parent_id in the API at least. (It would be great to finally finish this, and replace the old general comments package with it anyway.)
