Colm,
What are you going to authenticate? If they can use any username/password, then are you just checking whether a port is active or inactive?
You're probably talking about defining a new authentication module (which can then do anything you want). A quick way would be to use rlm_perl to embed a perl module that does the authentication (see
http://wiki.freeradius.org/index.php/Rlm_perl). It can access a DB etc if required.
Thanks,
Alex