OP here. I don't know of a feed that does this.

However, something like that should be relatively simple to build.

HN's API (https://hn.algolia.com/api) can access all comments in a post. Should just be a matter of collecting all top-level comments, sorting by points, and sticking the top few into the RSS <description> element.

I'll consider adding this as an optional feature in hnrss. Would need to add caching, though, as each feed request would generate 20 separate API requests.