King Sejong the Great was a great believer in science and technology, and who had a dim view of hidebound Confucianism. He invented hangul to help educate the poor masses and encouraged the work of Korean inventors and astronomers, elevating those of low birth despite the protests of the elite. He was a keen military moderniser and supporter of literature.
It seems to me that King Sejong would have been the most receptive Korean leader to European ideas and sciences, but he was perhaps born a few centuries too early for Jesuits or explorers. WI an ATL European explorer with knowledge of the sciences had arrived in Korea during the reign of King Sejong. Given the great distances and the natural isolationism of Korea's elite, how likely is it for at least some western ideas (or at least some greater curiousity and understanding of the far western civilisation) to filter in?