To my credit, I tried to work this out myself, but could not find conditional tags that would work, plus the developer resources part of the codex is under construction, so finding code is harder.
Basically, what I want to do is if there is no replies in a topic, a message should display, something like
"There are no replies here, but you can post the first...".
It should kind of look like what WPMU DEV uses,
"No Responses Yet".