Hi Joshua, this sounds like Slack’s Multi-Channel Guest and Single-Channel Guest user types, mentioned here: Permissions by role in Slack | Slack
and they work like this:
Multi-Channel Guest: " These accounts can only access selected channels, and they can start direct messages with the people in those channels. "
Single-Channel Guests: “This type of Guest account can only access a single channel and start direct messages with the people in that channel.”
Sounds to me you’re looking for Single-Channel Guest? Perhaps it’d make sense to add those user types — if they’re useful in Slack, then perhaps in Discourse too.
(Perhaps a tips, then, for Discourse developers or plugin creators, would be to have a look at how Slack has solved this :- ) )