Invoke A Forms Auth Login Modal Dialog Instead Of Redirecting To A LoginUrl Signin Page?

I've just finished implementing a modal dialog login popup for my ASP.NET website. The experience is similar to what you get with [URL]. When you log in, you get a fancy popup modal dialog (provided by the JQuery Tools Overlay control). The dialog is an ASCX file in the Master Page, so it's available globally. It uses a PageMethod to validate the current user. If the validation succeeds, I call window.location.reload(); in the PageMethod's success callback.

This works great when logging in on a page that doesn't require authentication, but what about when non-auth users are trying to navigate to a page that requires auth?

Is it possible to modify my web.config file so that instead of redirecting to a SignIn.aspx page for non-authenticated users I simply invoke the modal dialog instead?

Let's say a non-authenticated user is on Default.aspx which doesn't require auth. He wants to navigate to "Add.aspx" which does require auth. What's the best way to handle this with a modal dialog popup?

If I have to use a dedicated page, I guess I'll just have a SignIn.aspx page that invokes the dialog when it loads and if authentication succeeds, it'll use JavaScript to redirect to the destination page.

But ideally, I'd like to do the login from the Default.page and then redirect to "Add.aspx" with script.