You installed py27-letsencrypt from ports? I posted a command in here that I used on FreeBSD and it worked find to register a certificate without disturbing the running web server. The webroot authenticator does work in FreeBSD.
[1]: https://letsencrypt.readthedocs.org/en/latest/using.html#web...
Maybe I'll just write a firewall rule to redirect traffic from letsencrypt IPs over to the standalone client.