I could use a conceptual push in the right direction. In my U.S. state (Wisconsin) sales tax is calculated by county. Zip codes almost always cross county boundaries, and larger cities frequently do as well. So those aren't good options for calculating sales tax.
So, much like WC 2.0 has added cities to the tax table, I'd like to add U.S. counties.
I'm wondering (in general) what it would take. I realize adding custom fields to a form is easy via an action and function. But it seems integrating a county tax option into the mix would involve a lot of hacking the core WC code to make it work.
Any general thoughts or advice?