authenticating based on Nas-Port-Id

hi, i been using freeradius fastuser based authentication for several thousand adsl customers for the last year or so now and it has proved extremely reliable. in order to simplify customer setup and minimise lost password support etc id like to start authenticating users based on NAS-Port-Id rather than User-Name. (the dslams/basen we use support populating NAS-Port-Id with the dslam/frame/slot/port). i am also working on migrating to a mysql authentication backend. so, does anyone have any tips on setting up NAS-Port-Id based authentication with users/fastusers/sql? i hope to permit the user to use any username/password combination. thanks for your help and an excellent radius server, colm