Avoid losing PostBack user input after Auth Session has timed out in ASP.NET

// Global.asax
void FormsAuthentication_OnAuthenticate(object sender, FormsAuthenticationEventArgs e) {
   // check for postback somehow
   if (Request.Url == "MyPage.aspx" && Request.Form["MySuperSecret"] == "123") {
      e.User = new GenericPrincipal(new GenericIdentity(), new string[] { });
   }
}

( HttpContext.Request.UserHostAddress )