I’ve actually been working on a save manager for GD that will allow you to keep your mods and mod characters all separate from each other - in other words, only one mod and any characters you associate to that mod will be viewable in the in-game menu at a time:
Eventually, I will be adding in “storyteller” functionality that will allow you to add both screenshots and text to a character (just in the app, of course). I am unsure at the moment tho how long until I have it release ready. Been on a little break from it after the initial coding frenzy getting the base functionality in a usable state.
Other than that @MikeHeydon 's tool - GDViewer - mentioned above allows you to add text/note’s to a character already and would enable you to do what you’re asking. Well, with the exception that this can’t be done “in-game”.
[Tool] GDViewer - Fast lightweight character viewer and note keeper