I see your predicament. But instead of the potentially messy solutions discussed so far, what if the topics are indeed created normally, but until they have a reply, they are unlisted, i.e. not visible in the forum stream and only accessible via a direct link. Would that be an acceptable compromise?
In other words, the basic spec request becomes something like this:
- Option to set topics generated for embedded comments as unlisted by default.
- Special flag that can be put on selected unlisted topics (in this case automatically) to make them unlisted if they receive a reply.