A user recently expressed a desire to process message codes on a 'forwarded-to' page. This is possible with some PHP that loads the URL, grabs the HTML, processes it and outputs it. The only drawback is signup.php would be shown in the address bar instead of the forward URL.
Almost the same thing could be achieved with a ListMail option to read a URL once then automatically place the HTML right in the Custom HTML 'data' section for editing.
