@Tat has done this on XF (feeding into @faraday's +cookie system) because she fell in love with the +comp system on the now-defunct Second Pass. Where it was, indeed, just a way to leave nice notes for other players and let them see them after the fact.
Tat can talk more specifically about it if she ever wanders back, but basically every character gets a Comp object and all the comps for a player are stored there. Of course, I know there are limits for how many attributes a single object can hold, so I guess some day we may run into that.