Yes, you're looking for a "captive portal" or "walled garden". This really has nothing to do with FreeRADIUS.
See <http://www.personaltelco.net/index.cgi/PortalSoftware> for a mix of commerical and open source solutions. --Matt - List info/subscribe/unsubscribe? See http://www.freeradius.org/list/users.html
