What if it was a decentralized system with authentication? Could a community of users host content internally, and add a users public-key (of sorts) to some global "allow" list?
There is a well known decentralized system that scales called email. It has proven difficult to keep clear of spam, but that isn't an issue on a closed/authenticated email network.
A Sybil attack can be seen as either a censorship problem, or a trust problem. As long as you are careful how you add new members, such a system wouldn't be at all vulnerable to a sybil attack.