>Is it possible to have a counter setup to achieve this?

Yes. It is.

>I'd like to know if someone has implemented realtime upload/download limitations and what methods were used.

Realtime traffic accounting would have to be supported by your NAS.  Any kind of traffic/bandwidth limitations has to be supported by you NAS, you have to tell freeRADIUS what data to store and how to calculate the values.. and of course, what attributes should it answer to NAS !

Regards,

E:S