I imagine I need a new KMS host key for Windows 10 to work but with Windows 2016 still months away I'm not quite sure how this is supposed to work. Whatever key I install would still need to be able ...

OK, I've got a 2008 standard box (not R2) with a KMS host key installed and activated. "slmgr /dli" on the server reports that it is using our VOLUME_KMS_B KMS key. It's activated, happy, good to go.