Here is a Chrome extension for the Twiteratti / Chromies here. “Whodis” which is an alpha release, lets you add private notes to Twitter profiles.
You will need to sign in using your Google account to start adding notes. I stopped at this point. I was not comfortable setting this up just to test the extension. If you are a heavy Twitter user who could use a feature like this, please review the permissions before proceeding.
It would’ve been nice to see the extension stored the notes locally and gave cloud sync as an optional feature, with login.