@pout Interesting idea, but the problem I see is the notes content might want to be kept private, and if you put it into the mail header then everyone could see it. Adding a option to mark private to keep it out of the header would just complex things.
@Michal_Burger mentioned in a post (Sync QuickText and Signatures across computers - #3 by Michal_Burger) about using a service for wider 2 way sync, maybe this is something that can be added to that.