I guess this is more than a start. The more I think about this the more I like the idea of allowing schema additions. This could possibly solve many problems at once: Keeping CSL core specs simple and maintainable while at the same time giving more options to those who need them. This could be especially useful for certain disciplines with rather special demands, like law, classical philology, biblical studies, film studies, musicology. I don’t think this would necessarily entail convoluted GUIs. It might be reasonable to have users who need this use the extra-Field in Zotero for example. (Of course, the idea that GUI tools could add fields if required by a style sounds intriguing.)
I don’t know how difficult it is to implement this. But if is feasible I am absolutely in favor. @cormacrelf Have you already thought about this?