Could you check "ip admission..." command in your environment? I think it can help you for doing this...more on:
http://www.cisco.com/c/en/us/td/docs/ios/sec_user_services/configuration/guide/12_2sr/sec_securing_user_services_12-2sr_book/sec_cfg_authen_prxy.html#wp1054547
You could post some text on your html page of interest and then maybe user could only just click on something and then proceed to normal Internet pages...
HTH,
Dragan
HTH,
Dragan