mod_osso REMOTE_USER CGI variable

I have a simple application authenticated using mod_osso and protected using static directives. I can see all mod_osso headers being passed to my application. In this setup, I am able to use mod_rewrite and mod_headers to make available the REMOTE_USER CGI environment variable as a custom HTTP header. The configuration for this is as follows:
<IfModule osso_module>
    OssoIdleTimeout on
    OssoSecureCookies off
    OssoConfigFile osso.conf

    <Location /app>
        AuthType Osso
        require valid-user
    </Location>
</IfModule>

<Proxy *>
    RewriteEngine On
    RewriteCond %{REMOTE_USER} (.+)
    RewriteRule . - [E=RU:%1]
    RequestHeader set X-Forwarded-User %{RU}e
</Proxy>

The rules are in a <Proxy> block because requests are being proxied to my application using mod_proxy. Things work fine in this configuration.