You could theoretically just loop through every community via "get_communities" and then
com_obj["community_view"]["community"]["hidden"] :: boolean
*I'm writing this on memory, the json structure may be slightly different
and then just subscribe to every community that pops up.
It will likely be slow though, and it's mostly NSFW + !news@lemmy.world that would pop up on c/all if you did. I also *think* lemmygrad as an instance is hidden, so those communities may pop up if they tend to reach c/all.
I'll discuss with the team about making a public list of hidden communities.