Good to see some movement on plan hints in postgres, but I can't get behind the ergonomics of stashing plan advice via query ID. Comment-based hints can be declared in the application, which is typically when you will want to update the hints when modifying the query. It's just a much simpler workflow and one which has a lot of precedence in other DBMSs.
The infrastructure is intentionally pluggable to facilitate different viewpoints on questions like this. I hope we see various add-on modules pop up that fetch advice strings from various possible sources.