Thanks for the email. The questions isn't so much whether coreboot supports Windows. The question is: Does coreboot keep up with all of the bios updates that come down for windows-related functionality for all of the motherboards they support. I imagine this would be pretty difficult for them to do, but I could well be wrong.
I've passed your email on to our R&D department. They try to contribute to the work of third-party developers if they can.
System 76 Technical Support