Yes variable , you can make it , look what i do to make it work with me in radius :
i insert the following lines into the file
/usr/local/etc/raddb/users
DEFAULT Auth-Type = Local
Exec-Program-Wait = "/usr/local/bin/mtauth %{User-Name} %{Calling-Station-
Id}"
and i wrote that perl script mtauth (mikrotik auth ) that check every username for mac address from that attribute ( calling-station-id) it's related to mac address all work fine like i want , configure that and try to make a script that make it for you