slipstream and freeradius Reply-Message

I want to be able to specify a Reply-Message attribute which is passed to slipstream, which in turn picks up an replymsgs file. However, I only want this to happen when the login fails. Everything I do seems to make it return a Reply-message only when it is successful. I am using freeradius, mysql and slipstream 4.0.127. I have basic authentication working ok. What I am trying to do is create custom failure messages for each user <http://www.nitroglobal.com/clickthru.cgi?id=outside> Nitro Web Accelerator works in conjunction with acceleration servers on the Internet to provide dial-up users with the ability to rapidly access Web pages and e-mail, and to download files much faster than would otherwise be the case."