The way I understand it, correctness of answer should be decided by the community not moderators: Is it in the Moderator's Job Description to patrol for correctness?. So I am not sure whether custom flag is a good way to go. NAA flag is a bit different, because it puts a question into low quality review queue: What happens when I flag a posted answer as “not an answer”? In any case, it would be good to know what moderators think about your proposal. — Martin Sleziak 1 min ago