You now need to teach Apache about the new custom template directory. Edit /etc/apache2/sites-available/eg.conf and add the following <Location /eg> element to each of the <VirtualHost> elements in which you want to include the overrides. The default Evergreen configuration includes a VirtualHost directive for port 80 (HTTP) and another one for port 443 (HTTPS); you probably want to edit both, unless you want the HTTP user experience to be different from the HTTPS user experience.

<VirtualHost *:80>
    # <snip>

    # - absorb the shared virtual host settings
    Include eg_vhost.conf
    <Location /eg>
        PerlAddVar OILSWebTemplatePath "/openils/var/templates_custom"
    </Location>

    # <snip>
</VirtualHost>

Finally, reload the Apache configuration to pick up the changes. You should now be able to see your change at http://localhost/eg/opac/advanced where localhost is the hostname of your Evergreen server.