I also used to do this in a couple of small startups. I loved having the review history just stored in git, and how commits that fix a problem also delete the comment about it, so when reviewing fixes I make sure every newly deleted comment is taken care of, then git grep "CR:" and if it comes back empty we're good to go.

However, I got a feeling it wouldn't scale well to large teams. Specifically because you have to manually keep very strict about not mixing reviewed and unreviewed code, and if you do it's hard to disentangle them and decide what's in scope for a review.