By using either FreeRADIUS or another RADIUS server you can use the MAC address authentication by creating an entry in which the username and the password are the mac (without : or – characters) you want authorize. Zeroshell manages the authentication by using Kerberos 5 backend that not allows to set the password equal to the username. For this reason if you want this feature you have to manually add the entry in the file /etc/raddb/users.
Regards
Fulvio