Is it good enough if the text is in the json but not displayed on the page? It might not be hard to modify the Discourse Spoiler Alert plugin to reveal text only to members of some group or staff. If someone wanted, though, they could look at https://meta.discourse.org/raw/12650 to see the hidden text.
Making it really available only to some users would be a good bit more difficult, as it would require something more complicated, perhaps encrypting the text and providing a decryption key in the json load only of moderators.