The addon should prevent any tab which is created in the background (aka. unfocused) from loading until it has been activated (aka. focused) by the user.
So if the browser starts up and the pinned tabs arent initally focused ... it should prevent them from getting loaded. Maybe try it and let me know if it works. I havent tested it explicitely myself, just stumbled over your idea/request and thought it might work for your usecase.