i think the answer. in a microblogging-compatible paradigm anyhow, is creating a second, evil kind of tag, where opting into discoverability is an intentional choice (perhaps keeping Hashtag as the discoverable version, and the new tag as the non-discoverable version)
these new tags would be out-of-band ONLY, and just as a cherry on top, let's make them a tree as well, so you can tag a post with, say,
tv show.season 2.spoiler and it would additionally get categorized under tv show and tv show.season 2