Fundamentally the problem is we expect Discourse sites just to have 1 domain. The reason for this is that we send out emails and have to pick a domain we can not pick random.
I am not following why not have the onion address as your hostname.
If you really really want to enable this ninja I am allowed to have 2 domains going feature, you can use a plugin, example of a commit that made this work for my blog is here: