Difficult, since at that time Spain was already under Bourbon Salic Law, and they were plenty of male heirs to get the throne.
Best bet: some Spanish Liberal tried to offer to Pedro I of Brazil the crown of Spain between 1826 and 1830 (before the birt of Isabella) and there were even some other proposals until 1833, when Ferdinand VII died. IOTL he never paid attention to them, because he was busy securing the monarchy in Brazil and the throne of his daughter in Portugal. Make him accept it ITTL, but I just can't see how would he be accepted by the Spanish.