How can I get sound working on Bootcamp + Windows 8.1?
Forget EFI and install using boot camp assistant.

After backing everything up, wiping my partitions, and installing Windows 8.1 using Bootcamp, everything works as normal.  The machine boots using bios mode, Cirrus Audio was properly installed after a single run of setup.exe from the Bootcamp support software, and the device is recognized properly as VEN_1013 DEV_4206 --Cirrus Audio CS4206B (AB 82).

It sucks, but it's worth it.  I plan on resizing windows' ntfs partition and booting linux beside it, but that's a problem for another day.