I can't imagine a modpack having any real grounds for copyright, which is what a license is built upon. Maybe you could argue that this exact combination of configuration settings would count? You could definitely copyright something like the quests from Agrarian Skies, or even all the extra Minetweaker recipes. You can also get the rights to a name pretty easily, such as Feed the Beast. If you want to set yourself apart as providing an 'official' version of a certain modpack, your best bet would be to own the rights to a certain piece of art, be it splash screen or texture pack or music file or something, and distribute that item with all 'official' versions of the pack.
However, I'm pretty sure we've gone well past the scope of this thread. This is really meant for in-game mechanics, not real-world legal issues.