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 -
The sense of "royal" was originally used in reference to gold and silver which were considered exclusive property of the Royal Crown as explained by the ENCYCLOPÆDIA BRITANNICA:
Royalty, in law, the payment made to the owners of certain types of rights by those who are permitted by the owners to exercise the rights. The rights concerned are literary, musical, and artistic copyright; patent rights in inventions and designs; and rights in mineral deposits, including oil and natural gas.
The term originated from the fact that in Great Britain for centuries gold and silver mines were the property of the crown; such “royal” metals could be mined only if a payment (“royalty”) were made to the crown.
Late Middle English: from Old French roialte, from roial (see royal). The sense ‘royal right (especially over minerals)’ (late 15th century) developed into the sense ‘payment made by a mineral producer to the site owner’ (mid 19th century), which was then transferred to payments for the use of patents and published materials.