Why not leave the reason as a comment instead? I don't really see a need for a new feature.
I expect HNs algos could spot such a comment thread quite easily (thereby avoiding what you've described). That doesn't require a new user feature.
i) Just downvote a comment that you think should not be on HN but which is not bad enough to flag
ii) Get involved in a tedious sub-thread bit of bickering with the comment's author about why you think they're wrong
iii) Write a post explaining why you think they're wrong, and have them reply with clarification, and you both end up agreeing and happy.
iii rarely happens. ii happens too often, and i is preferable to ii.