Have you seen the request getCommentsByTagCreated for comments with certain tags? But I think only the tags explicitly set in the comment are taken into account there too.

Thanks! I didn't look at it carefully yet, but I think I experimented with that a few days ago. Maybe the challenge was that it only looks at the first tag(?). Not sure, I will look at it more carefully.

Did I understand you (and @danmaruschak) correctly that you want the comments on the posts with a specific tag returned?

Exactly, if I'm following the "science" tag, and someone uses that tag in a top-level post then I want to get notified of the post and all replies below the post - regardless of whether or not the reply also used the "science" tag. Ideally, I'd repeat that process with tags in lower level posts, but I think that might require too much computation.