nginx per vhost documentroot + tag insert

vhost.conf

server {
        listen   80;
        server_name ~^(www\.)?(?<siteid>.+)\.domain\.com$;
        log_format timed_combined '$remote_addr - $remote_user [$time_local]  '
                                    '"$request" $status $body_bytes_sent '
                                    '"$http_referer" "$http_user_agent" '
                                    '$request_time $pipe';

        access_log  /var/log/nginx/static_sites.domain.log timed_combined;
        root /var/www/$siteid;

        location / {
                root /var/www/$siteid;
                sub_filter      </head> '\n<!-- insert tag for $siteid -->\n</head>';
                sub_filter_once on;
                index  index.html index.htm;

        }
}