Spitballing for people regularly adding extra files (and @geek1011):
Does a source: target config like what @jcn363 proposed makes more sense than simply being able to simply feed another KoboRoot and letting the patcher merge them?
(It's a minor tooling change on my end, so I'm not particular about the details. I kind of like the merge the tarballs approach because it seems like a bit less work for me, and can be re-used for complex things more easily, but really, both approach would work for me
).