Have you thought about using an httpHandler for this?
In your web.config, register the handler:
<system.web>
 <httpHandlers>
   <add verb="*" path="PageBuilder.ashx" type="YourNamespace.ClassName, YourNamespace"/>
 </httpHandlers>
...
You can put whatever logic you currently have for building the aspx in your handler:
//use the IRequiresSessionState if your handler requires access to the session
public class PageBuilder : IHttpHandler, IRequiresSessionState 
{
    public void ProcessRequest(HttpContext context)
    {
        //logic to build your page
    }
}
Simply point your redirects to PageBuilder.ashx and pass the data either using querystring variables or with the Session object.
You can learn more about handlers here:
What is an HttpHandler in ASP.NET