I think I'd keep '@' to id a feed, and just append something else to the other end .

Ack on that, analogously to message cypherlinks.

Another pretty valuable replication option enabled by offchain-content: Only fetching the content of messages with a certain type. That way, my non-coder friends would not have to download all the data that e.g. a package manager app could add to my feed. They might still choose to do so anyways, but having the option to opt-out in resource-constrained settings is nice.

An advanced mechanism you could build on top of this involves all applications registering which types of messages they deal with at the server, and the local server then only fetching those. Or if storage overhead is not a problem, it could still fetch all others, but only after the directly useful message contents have been fetched, improving perceived performance.