I’m hoping that this will “just work” when Mastodon gets quite-posting. You could take a Mastodon post, and then quote-post it into a community by mentioning that community’s name.
This would create a separate thread of replies, which is good. A person shouldn’t be able to suddenly thrust a bunch of community replies onto someone else’s post. So basically it’s what quote-posting is for, but sharing it with a community instead of just your own followers.
I’ve found that, currently, this kind of works and kind of doesn’t. I’ve boosted a few lemmy and mbin comments from my Mastodon account, and it shows up in feeds just like you would expect it to. Unfortunately, the parent post of the thread only shows as a link to the lemmy/mbin thread, rather than showing the full text of the original post. So it’s hard for people to see the context of the comment.
Mastodon appears to see lemmyverse comments the same way it sees Mastodon comments, but the top-level post that started the thread is somehow different.