Ok, I think I knwo what you mean. What the payment gateways do can be achieved easily in PHP/Cold Fusion and I think with an extension in ASP.
Basically the page that is called on the payment server does a call to your script (the silent post) for authorization then sends a response to the client. In the 'silent post' the server is acting a s a client to call another page, either on the same server or a different server.
However, although this is done behind the scenes, this is still in response to a request by the user and at the end there is still a page returned to the user, so they have a slight advantage compared to what you are trying to do, where you say you want the browser not to get any response?