I’ve submitted a feature request to Github Add extra 'Difficulty' labels for Puzzles · Issue #1239 · online-go/online-go.com · GitHub. I don’t really understand which parts of the site are open source and which aren’t (not that I necessarily have the ability to implement it), but that should increase its visibility, and if it’s easy to implement maybe it’ll happen soon (or even sooner if it ends up being front end).
Feel free to read/comment the suggestion there, and I can edit what I’ve written if you’d rather keep the suggestion there clean and keep the discussion here.