I noticed that it's possible to have much longer entries with custom headings in the player's notebook if you edit the text file directly in your save folder. The only limitation is the screen size (if your note is longer than the screen can display at one time, then scrolling down will skip to the next note). Editing the file is straightforward for anyone experienced with arrays or node lists. The only caveat is that you may need to take care with the length of each line as I doubt that word wrap is supported (I never bothered to test that).
I think that a frontend utility written in a cross-platform language like Python would make notebook editing more accessible. The developer could specify the notebook data structure in an XML document and then use XSLT to convert the user's input to the text file.
No, I'm not going to make this myself, because I have never written a line of Python in my life.
