Using RewriteFilter defined in web.xml within application

Hi all,
In my web app I'd like to use request.getRequestURI() to dynamically
construct URIs for various redirection purposes, but I'm
inconvenienced by the fact that my app makes use of a RewriteFilter
and request.getRequestURI() returns the true servlet's URI as opposed
to the pre-rewrite URI used in the request.

Am I missing an obvious way of making use of the RewriteFilter within
my app? I've looked at com.caucho.server.webapp.Application for any
hints in grabbing the filter from the context, but I'm stumped.

Here's an example of a workflow in my app where I'd like this to work:

1. Unauthenticated user attempts to access a restricted resource
2. Servlet redirects user to login page, appending original request
URI as a query parameter
3. User logs in, servlet redirects to originally requested URI via the
query param

Thanks for any help,
Tim