I'm agitating for some fields to be added to the core for any mods to use and maintain some consistency.

If you already use a tax id handling of your own devising (as I do) or if you see that need approaching, I would suggest you read this discussion on Github to see what is already happening in the EU:

https://github.com/zencart/zencart/issues/3058


I have closed that thread to continue it here for better visibility to users rather than developers.

I am interested in what scenarios exist, so conclusions may be made about the most appropriate start point, for what (certainly in the EU) will be obligatory in the near future.