BinhQuang "Private discussions" is on the roadmap (it's the last item under "Big Picture) as a planned feature.
I'd guess Toby is planning on implementing it in fashion similar to private conversations in esoTalk. In esoTalk, the discussion creator can invite any number of users and/or groups to participate. This is very similar to what you're proposing, but somewhat more flexible.
It seems possible that your proposed extension could end up being obsoleted by such a bundled extension, which would be a shame. So you might want to get together with @Toby and talk about how you can contribute to the Flarum project, to avoid duplication of effort.
