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