Spoiler for certain user groups?

At Discourse, we link to encrypted pms for this kind of thing. The PMs are treated as wikis. So we would have a regular topic:

Super secret information

You will need this when responding to @terraboss on Meta. Click to view

And then the link would take you to the encrypted PM. You can control access to the PM per-user, or per-group if you don’t need encryption.