You can do that with another workaround:
Make all your categories only visible to trust_level_0 (or tighter) except the one which contains your info posts (make it visible to everyone). And then make your forum public. Of course, they won’t land on the login page any more and this is a pretty radical way to achieve that goal.
Or fix the underlying problem and wait with me for a solution to the other bits I linked to above (Making the About and FAQ visible to anon)