Nginx reverse proxy with client-certificate authentication

I understand I can have nginx to check client certificate before forwarding requests; something along the lines:

server {
    listen 443 ssl;

    server_name my.server.com;

    ssl_certificate     /etc/ssl/my.server.com.crt;
    ssl_certificate_key /etc/ssl/my.server.com.key;

    ssl_verify_client       on;
    ssl_trusted_certificate /etc/ssl/myca.pem;
    location / {
        ...
        proxy_pass http://my.backend.com:9999;
    }
    ...
}

Question is: can I extract certificate details (essentially client certificate Subject:CN) and use it to modify forwarded URL (e.g.: adding a fragment to it)?

If that's possible, how?