Jonas - keep your cool.
Better lose a few days, or even weeks, rather than screw up with your first release. Becoming hated by the customers, or releasing something going to be pirated all at once, both options would be bad. The most "affordable" thing to lose is time ;)
I think I once saw a WP plugin not working by default, requiring a login-password provided by a third-party website. You could install and even activate it, but it wouldn't do anything unless a login and password were provided in that plugin's settings page.
I don't know how this was coded, I don't know if the plugin code was protected, sorry but that I cannot say. I can only imagine a part of the plugin's code was a huge ASCII soup of base64 characters, and the key was obtained from a remote server when correct credentials were given.
As for the licence, GPL seems out of order. I'd recommend you find other commercial plugins and look at their own distribution licence.