Failing that ... this looks like an interesting hack that might work!?
https://moodle.org/mod/forum/discuss.php?d=325615
I'm not sure how the functionality will act when it comes time to "manually enrol" users in these different hidden categories.
I haven't tried this myself but you can try playing around with the view participants capability as I think that is essentially what you are looking for? I'm not sure how that would work, if at all, at the category level.