I don’t think Safari 6 will work even when you solve the TLS issues by adding additional cipher suites.
You can add missing cipher suites by overriding the nginx config file. Add the following snippet (untested, but it should work) to the hooks section of app.yml and change the value of ssl_ciphers to your liking.
after_ssl:
- replace:
filename: "/etc/nginx/conf.d/discourse.conf"
from: /ssl_ciphers .*/
to: ssl_ciphers <your_complete_cipher_list>;
BTW: I’m trying to add support for Elliptic Curve certificates to Discourse which would make it work for IE11 out of the box.