I am, too, trying to figure out how to customize Map Notes. I looked at the previous post you linked, and opened the online explorer like he suggested, but it seems to do the exact same thing in there. In addition, the online explorer is not compatible with the newer maps from ArcGIS Online, so you can't edit a web map you have already made.
I have recently started messing with config files for custom app templates, and one would think something similar would be available for web maps or at the very least map notes, but so far no luck. I am commenting to follow this thread in hopes an answer will come!
For anyone from Esri looking at this, we need to be able to show only placed items in the legend (not everything available in the notes template) as well as the ability to edit the label for the symbols in a package. For instance, I only need 3 symbols in my map notes, and the "Citizens Request" template works well for what I am doing, but I would like to change the labels that appear in the legend. This seems to be something saved in the web map and cannot be configured in the app, so it would be a very nice feature to incorporate in the near future if there's no roundabout way to do it already.
