Challenge-response with mod_auth_radius

Hello, We would like to use freeradius with a 'home made' challenge response authentication scheme (we will build our own module) using mod_auth_radius. Ultimately we would like to prompt the user (after successfull authentication) with a challenge that he would have to enter in a securID like device and enter his response in the mod_auth_radius prompt. I've been toying with freeradius and mod_auth_radius for a few days now, and I still haven't succeeded in prompting the user with a challenge from the RADIUS server. Could anyone point me in the right direction ? Is it possible ? We're using mod_auth_radius as a basic client, but we could use any type of NAS. Best regards, Arnaud