I think this should be done as a plugin.
So when you create a forum topic, it has a plugin macro in its header (or footer, depending where we want to render the "Link to FAQ" button). That macro renders a button and when you click that button, you get a list of directories you have write access to. You pick the directory and specify a name for the "link" document. That document is then created in the target directory and it will have a [<=documentRedirect] macro in its body. When the WikiTextHandler tries to render that macro, it instead sends the browser a redirect to the original forum document.
That way we avoid duplicating content but we still need to handle deletion. We could just let the redirect run into a 404.