Apache Rewrites in VirtualHost Changing Rails Base URL?

I'm trying to configure my Rails application to handle most requests
using Mongrel with a select few (file uploads and other long-running
processes) passed through to the CGI dispatcher.

Here is what my Apache configuration (httpd.conf) looks like:

    <virtualhost example.com>
    ...
    # Long-running processes
    RewriteCond %{REQUEST_URI} ^/lorem/ipsum.*$
    RewriteRule ^(.*)$ /dispatch.cgi [QSA,L]

    # Redirect all non-static requests to cluster
    RewriteCond %{DOCUMENT_ROOT}/%{REQUEST_FILENAME} !-f
    RewriteRule ^(.*)$ balancer://mongrel_cluster%{REQUEST_URI}
[P,QSA,L]
    SetEnv force-proxy-request-1.0 1
    SetEnv proxy-nokeepalive 1
    </virtualhost>

What I find is that a request to /lorem/ipsum is properly passed
through to the CGI dispatcher, but that request is processed as if it
were merely /. As such, it always processes the main page of my
application instead of /lorem/ipsum. All the URLs in the source are
also prefixed with /lorem/ipsum, including linked media such as
stylesheets and javascripts, which makes me think it's treating /lorem/
ipsum as if it were the RewriteBase.

It's probably worth noting that I've tried using CGI from inside
the .htaccess configuration, and this never happened. But in order to
make my .htaccess file environment-independent, it'd be much easier to
keep the rewrites inside httpd.conf (since that's where the proxy
redirect is), so each environment can be customized as necessary
without some overly elaborate deployment/configuration scheme.

Is there something about my Apache rewrites that is causing Rails to
misinterpret the request URL and treat it as / instead of /lorem/
ipsum? Why else might Rails be treating the URL differently in CGI
versus Mongrel?