Git Appraise uses notes to store code review data: https://github.com/google/git-appraise

SubGit (an application to mirror subversion to git) is using git notes to attribute git commits to svn revisions: https://subgit.com/book.html#client_config

If it's just an experimental branch, what is the reason not to commit?

Are the notes tied to a specific commit, or is it per object?

A note on a commit is stored as a text file, named with the commit id (git sha). The file is revision controlled by git, usually under refs/notes/commits.

You can 'git checkout refs/notes/commits' and see the structure.

actually i think it would be nice also push it to remote repository..

Seems possible, just not straightforward: https://gist.github.com/topheman/ec8cde7c54e24a785e52

Hadn't heard about this feature, but as I understand it, the notes are stored as commits, just not in the same namespace/on the same branch as you are working on.