I would suggest a sidebar hook that shows topics with no replies at all for the next version of IP.Board.

Whilst it's not exactly what you're looking for, as a workaround in the meantime, on a forum by forum basis you could use the filter options to sort by Topics;Open, Order:Replies, direction A-Z, then your unanswered posts will be at the top of the page.

If you don't want to wait: http://community.invisionpower.com/topic/299372-is-there-a-possibility-to-show-all-postthreads-with-0-replies/page__view__findpost__p__1882316