Hello,
Typically we don't proxy MAC authentication to the back end NPS RADIUS server. In a typical deployment MAC authentication is handled locally, and the NAC is designed to auto accept any MAC authentication request regardless of password, username, or even RADIUS shared secret. MAC Authentication is used to identify the end system, more than as an authentication mechanism.
We do have a few customers that use NAC to proxy the MAC authentication back to NPS, but there isn't much known regarding what their configuration is. I suspect they have users with either usernames of the MAC address, or an alias that serves as the username of the MAC address.
Thanks
-Ryan