It would seem to me that charging for premium support for libraries would be far more effective than charging for a license. You don't want to do anything to limit its wider use lest it be supplanted by a more widely available library that has a less restrictive license. But it's always painful to get support for very popular libraries, so that pain can be monetized quite easily. This also has the added benefit of locking people into using your library. They can either pay cash for premium support, or they can pay the tax of switching to a different library.