"My code is freely licensed open source, Based on GPL, with the addendum that Goldman Sachs can go stick their head in a pig"
As I understand it, there is nothing preventing this from happening.
It is surely open source. It's just not Open Source. An important difference because of the capitalization.
http://opensource.org/osd
So that even if they're speaking out loud, where one can't see capitalization, everyone knows what they mean.