@arthurattwell: See if this proof-of-concept plugin works for you. It's rough and there's not a lot of error checking, but if it works, we can talk about how best to proceed.
Right now, it works by selecting an external xml file with a file dialog and adding it to the META-INF folder. It assumes the XML file is already named correctly. If it's not, it probably won't function correctly. We can decide later what's best in that regard.
As always, Sigil shows the file in its 'Misc' folder, but it's definitely in the META-INF folder if you extract and look.
This plugin can now be found in its own thread in the Sigil Plugins subforum:
https://www.mobileread.com/forums/sho...d.php?t=272241.