This will depend mainly on getting a document number namespace into CSL. There has been discussion on this. No action so far, but it's on the menu.

This one will take some time to resolve (a year at the outside in a best-case scenario), for several reasons. At the moment, CSL circles are commencing a discussion of dates handling, which involves detailed decisions that span multiple projects. That is a separate issue, but in view of everyone's time constraints, it's likely to take precedence, until it's been settled. A second reason for a bit of lag time is that a flexible document number mechanism (think something like the "Pages" menu button in the Zotero integration plugins) will require changes to the CSL language itself, with consequent updates to documentation, to conversion/migration utilities, to the CSL schema, and to consuming applications (of which Zotero is one).

I have a strong personal interest in getting a flexible mechanism for document numbers into place, so it won't be forgotten; but it's likely to take awhile.

Yes, indeed there have been. The documentation to back it up is not out yet, but UN documents are supported by the legal styles built for use with Multilingual Zotero. The project is at http://citationstylist.org/.