I see what you are trying to do there, but I have a question. How are users supposed to enter the hidden topic, when that’s in a private category?
You mentioned:
That’s a good idea. You can create a topic, unlist it, and if someone replies, you can list it again. I am not seeing a way that it can automatically being listed, nor posting it in a private category.
I also found this topic, which might help you out: