Sorry. I just read they you can’t get it to work.
The thing to do is use the discourse_theme to generate a new empty theme. Then look for anther theme component that has a “extends component” section… Oh. I don’t think it’ll work. You’ll need your code to target and modify what’s already in the dom. Actually, I’m not entirely sure that’s right, as I’m still pretty bad at such.
I think you can do it with just css if you use that component that adds the group membership to the css.
But the raw markdown is available in the route that i showed in my last post. I don’t see how even a plugin can make it safe such that it will be impossible for people to see the hidden text. Something like user notes might solve your problem, or if you really need it to target posts, you could use user notes as a model to make post notes that would add a post custom field that only staff could see.