My understanding is that Andreas Palaiologos, the theoretical successor to the Byzantine throne, willed his imperial titles to Ferdinand II of Aragon and Isabella of Castile, but that the two never really utilised or aknowledged the titles in any meaningful way. Assuming that Spain would not look kindly upon the Ottoman conquest of Byzantium, and wanting to capitialise on the Fall of Granada, why would the Spanish never adopt these titles even in a symbolisc fashion. Considering that many of the other European powers, including a few of their rivals, were ready to spring on any even tangential claims to the title, why would Spain not even really aknowlege the act?