In a repository mode of editing (where you can edit several documents at once, some dependent on others in the same set), one could easily edit a complete XUL application. (I intend to create a demo of this as well.) Part of that could include transactions that affect more than one document at once. Here’s a sample XUL document I cooked up for adding a XUL menuitem, with support for localization and creating key and command elements related for this menuitem if the user desires. This user-interface would edit the master XUL document, a localization DTD, and possibly an overlay for menus.
Improvements are welcome! There’s certainly room for it. Send me an e-mail with your changes to this code; if it makes sense, I’ll add it. (Don’t worry about where the menuitem gets placed; I’ll be working on that.) Bear in mind this UI must fit into either a sidebar or a bottom bar (I haven’t decided which yet).
If you’re reading this via planet.mozilla.org or feedhouse.mozillazine.org, you’ll have to visit this article directly for the demo to actually work.