prenotazioni posti Spagna in Italia

Here are the known possibilities to make Spanish seat reservations : 

Not possible online because RENFE doesn't want to…

You can make all of them when you arrive in the country and availability shouldn't be a problem (no quota)

non si possono prenotare i posti nelle stazioni internazionali, per esempio Milano? O devo aspettare di essere in Spagna 

I don't think that it's possible but you could go there and ask. Let us know what you find out :)

In Milano maybe here you could do it:

Otherwise try your luck in Verona. DB agencies should be able to make it.

Indeed, Trenitalia can't do it, but any DB agency should be able to, if the staff knows how to book those trains. The most common error is that they try to book Spanish train with the timetable-based method, but that doesn't work. They should use the manual search using the train number.