As mentioned in the post, git notes can be used for code review, and I wish code review systems would have but a front end system that end up being encoded into the source control itself: think about all the times you had to find the first commit that introduced a specific line then trace it back to the code review that approved that change. It can surely be streamlined even when external to git, but since it's coupled, to me it makes sense that it should reside together and be available offline just like every commit.
Right, git-appraise seems to take that approach (I say "seems" only because I've never used it) yet it's the exception to the rule when compared to almost any other code review system for git that's out there. I'm sure there are good reasons for that being the exception but I'm also guessing that one reason being vendor lock-in.
