I know there is a connection between "proper" royalty (nobility) and intellectual property. The statute of Anne is often cited as on of the first incarnations of modern intellectual property law. It can be interpreted as a "royal" guarantee of intellectual property. Around that time, similar royal decrees can be found in France and other countries.
Although I feel that these circumstances might be the origin of the term "royalty" in the context of intellectual property, I haven't been able to find a source for this connection. Also past experiences tell me that the obvious answer is not always the right answer when it comes to etymology.
I'd like to find some primary or secondary sources as to when and why the word "royalty" had come to mean:
a payment to an author or composer for each copy of a work sold or to an inventor for each item sold under a patent - Webster Dictionary