Created a workaround for this Feb 7.
Had some correspondence over the weekend with Dieter Köhler (author of the OpenXML code I'm using to save and load files), and he confirmed that the XML 1.0 specification disallows characters in this range. The question now is how to handle situations in which people insert these characters. The XDom code is asymmetrical in that it fails to raise any error when saving a node containing illegal characters, but it does raise an error when trying to read them back in; I need to allow for this by somehow escaping these characters myself.
Posted time spent researching this and checking into my code. Also made it a task to add the relevant code to the app.