Again next to impossible. Bavaria was a second tier power who was only elevated to Kingdom by Napoleon. There would be no political, strategic or economic reason for the Queen of Spain to marry a Prince of Bavaria. Now a more interesting match would be to have Isabella II marry Infante Carlos, Count of Montemolin. He was as Carlos VII the Carlist pretender, and the idea was seriously considered. Hell its why his father "abdicated" in his favor in 1845, to try and unite the Carlist and Isabello claims. Such a marriage would more or less guarantee that Spain would have no more problems from the Carlists.