Does something like this exist? As far as I can tell, setting something similar up would require:
A dedicated Lemmy instance for Mastodon tag-based crossposting (probably, since most existing instances probably don't want to clutter their instance with thousands of script-generated communities)
Some kind of form on a site where users who want to follow a Mastodon tag that is not already set up as a Lemmy community could submit the tag they want added, which would automatically set up:
A Lemmy community for that tag on the aforementioned instance, and
A Mastodon hashtag bot that automatically crossposts all posts with that tag to the linked community
I'd see this to be implemented in Lemmy itself. Hashtags are a global thing, not instance-specific, and should already be available to Lemmy via AP.
Lemmy would "just" need the ability to display a hashtag as a "community" containing posts made under that hashtag. Question is what to do with replies but given Lemmys design, they could probably simply be left out because, if it's truly on-topic, the post will likely contain the hashtag too and therefore land in the hashtag "community".
I don't see why lemmy should leave out replies to top-level tagged posts. A tagged post is essentially like any top level post in lemmy. They could be displayed like any other lemmy post, with replies displayed in thread below
I was being a bit unclear here, what I meant was that replies mentioning a hashtag shouldn't show up as posts in its pseudo-community; only posts. Replies to a post with a hashtag would show up regardless of content.