Using forms authentication with URL Rewriting problem

I am using a third party url rewriting product called ISAPI Rewrite that interecepts the requests before ASP.NET and rewrites friendly urls to their aspx pages. The problem am having is with forms authentication. When a user requests a restricted page, forms authentication automatically redirects to the login page with the requested page in the "returnurl" query string argument. The problem is that the url at that point has already been rewritten to its .aspx form, but I need it to keep the original URL before its rewritten. Now, ISAPI Rewrite allows me to get the original url using a server variable, but from asp.net's point of view it has no idea what it was. Is there a way to override the forms authentication login page redirection and substitute the friendly url for the rewritten one in the ReturnUrl querystring argument?
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