Haha that was actually drafted by yours truly. So you know my game plan already.
https://github.com/nostr-protocol/nips/commit/a9f7058659171d8f4a0f788751a39ad78b95f557
I guess this isn't nip-86 yet but it looks like it would work for paywalled content. people without the shared key would see encrypted data or nothing at all... better would be to make it so they could see how to decrypt it
Discussion
Oh, maybe you already knew that. Sorry, memory like a goldfish. You could indeed use shared keys independent of closed groups