If editing the file out-of-game is important to you, I don't see the benefit of being able to edit it IN game. I mean, it's already dead easy to just have the .txt file open in another window while you play, what's to be gained from having an in-game text editor that can only load 1 file? True, having multiple "pages" is something that programs like Notepad don't offer, and multiple "pages" is the best way to illustrate things happening on different z-levels . . . but DF's existing Notes screen already does that, by literally using different z-levels.
What I'd like to see in the Notes screen is the ability to Copy & Paste designations, rather like a macro. Arranging things like bedrooms would be a breeze if I could just dig out & furnish one bedroom, make a new burrow defined by that room, and then move, rotate, & flip that burrow however I want, and then Paste that burrow as many times as I like. Furniture would be a problem, though: Would that in the Copied rooms have to match the material and quality of the original furnishings? What happens if the original room contains an artifact? Etc. It's a difficulty, but happily only a minor one.