I'm also looking for advice on busking in northern Spain this summer, particularly Zaragoza and San Sebastian. Any tips on permits/how worthwhile it is to bust these areas?

Our tips for Milan, Italy - my boyfriend made a fortune playing bagpipes in central Milan, near the cathedral and main square. You have to go to the police station close to Duomo/San Babila metro, give your details, pay a very small amount and book your slots for the week. They give you a paper permit which you can show to police (they do tend to check permits quite regularly but you'll normally get away without having one if it's the first time they've seen you, play the dumb tourist)