That's a good point about wanting something that will work in comments!

And that's also a good point about "spoiler" cuts turning into things unrelated to spoilers -- however the feature works out, it should probably be named by what it does instead of which problem it's designed to solve. Something like "strongly hidden" or "never auto-shown" or something catchier instead of "spoiler", so it's just as relevant to those other problems it'll inevitably end up solving as well.