Divert first web request after policy allow to a URI

Why don't you try to use an AAA HTTP to send the request to the webserver and do all the checks? You just have to check the response back from the web server and show the message with the terms based on that. You can use a decision message box to finish the session if the user doesn't agree. You can use another AAA HTTP to confirm that the user accepted the terms.

If that webserver is able to receive a POST message with the information you have from the customer, this should do the work.

Another option is to use an irule with sideband connections.