Ah! I see I mis-read/misinterpreted your original request.
I confess that I would tackle it a completely different way.
I never allow postings at the top level when I have sub-forums. I remove posting rights from the top level and then over-ride them again for each of the sub-forums - and, of course, I make sure there is a suitable sub-forum for the topics that otherwise might be posted at the top level.
The "Mixed", "Board" and "Category" apps are all designed to work that way and all appear much more like other forums that you might encounter on the Internet, so users find them easier to work with as well.
If you are fully committed to allowing posting at the top level, rather than having your first sub-forum reserved for those posts, then it would take some NAML coding to achieve. If you haven't got those coding skills - I haven't! - then you'd need to pay from Premium Support and ask Nabble to add the necessary code to your forum.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.