There is not a method to get the MAC address. We are discouraging the using any unique HW identifier in any form of user account linking or authorization. We would like a factory reset on the device to assure the user that all personally identifying information including user accounts on different channels associated with the box is removed from the box. This requirement is not met if sites are using the Mac address to associate accounts as it survives a factory reset.
If you refer to the Device Registration and Linking guide, we discuss a "rendezvous" linking method that is an easy way to associate the box with a user account on your site and is also an association that is broken when the user does a factory reset on the box.
I think that Netflix uses the boxes serial number.