@Ted I was thinking about that yesterday. It would be really messy though and would need a lot of work.
Basically you have your db that contains all your sub categories. You create x number of nodebb instances, each instance would be one category, then using the permissions in the ACP, give access only to the subcategories you want for that subdomain(category). Then on your main nodebb site, you would just create external links to each subdomained forum containing the subcategories.
The issues that I can see straight away is that you would need to log in for each subdomain you visited, unless you can manipulate this to log you in to all your nodebb instances. Changing between nodebb instances would connect to a new socket, which would make initial page load slow, as it's a call to a new site technically. And you would have to re-route the breadcrumbs so that Home became (Category Name), then a new Home made before that which took you to the other site. See screenshot at the end.. I'm sure there's other issues you'd run into with this too. Like each instance would need it's own theme, and it's own plugins. Unless rather than running a full instance of nodebb, you could just duplicate a couple of files like config.json and app.js etc, @julian will confirm if that's even possible, which would allow you to use the same node_modules folder, and the same theme etc, sort of like a wordpress multi site.
Having thought about this more, I'm not sure that would be possible, it depends entirely on where the permissions are saved, if you give a user access to a sub category on NodeBB instance 2, but not in NodeBB instance 1. If it saves to the DB, it will just use whichever permission was set last.