Web-based OTP authentication

Hi, I have a server which can authenticate a phone-generated counter-based OTP using a web-service (http or https). Can anyone suggest the easiest method for integrating that authentication method with the latest version of FreeRadius? I have looked at PAM and rlm_otp, but was wondering if there is a way in which I can call the web-service from a tool using exec-wait. My idea is to use the FreeRadius at the edge to receive the RADIUS requests and then pass the username and OTP using the web-service for authentication. Thanks in advance, Jai