I think thats actually solvable.
You could you have two keys? Your followers get a key and when viewing the content a positive check for the user key + follower key allows a third (stored) key to be added to the transaction signing.
Or perhaps just a check on user key + user is follower would work. This is just spit-balling. But i dont see an issue with multi-key signing to decrypt. It would however have several other downsides like usability, reduced public content (network effect), more difficult third-party app integration. Etc etc.
But the idea of encrypting content to be only viewable by some whitelist of users seems possible
Sort of, until your "private" content gets leaked or reposted somewhere. If you have a lot of followers you won't have any idea who did it either.
Not only could I encrypt it to a whitelist of users, I could encrypt some content with ABE, (attribute based encryption) so only users who have a certain reputation quantified on the blockchain can access it.