consider the cases in levels of complexity:
0) minimal base case for nostr is 1 person sending their notes to 1 relay.

1) 20 people white listed to a relay. Their feed it literally one relay and the people connected to it.

2) 2 communities with similar values sharing posts from relays

3) (nostr how everyone uses it) individuals with tens of relays to broadcast to and anyone at the intersections see posts.

The subreddit nip, whatever it is operates at level 3. That's fine but i think we should focus on 1 and 2.

Private groups can work with level 1 without cryptography. Anything more open is a gradient from level 2 to 3. 

If you just select a relay to post to for a given condition, thats not even a nip - the functionality already comes baked in with how nostr works right now. Clients just make relay selection an afterthought.