Well Salazar thought about bringing Duarte Nuno (of Kaiserreich fame) back to the Portuguese throne after president Carmona died in 1951, but decided to keep the Republic anyway. It's possible to designate him as Carmona's successor earlier, but he problem is that Nuno himself belongs to the conservative Miguelist branch of the family, making it less likely that the monarchy is restored by the time the Carnation Revolution kicks in.
Maybe the revolution is more broad across ideologies and skewed towards a more centralist approach (from what I heared, the revolution is skewed more toward the left, especially from the Communists since there was a pro-Communist coup attempt a year later) and decided to choose someone from the liberal, Brazilian branch of the Braganza family like Prince Eudes as a ceremonial head of state.
I think it's important to note the circumstances surrounding the restoration of the Spanish Monarchy. Juan Carlos I was groomed to be Franco's successor for years, and he spent that time feigning loyalty to Francoism while simultaneously meeting with exiles and opposition figures in secret. Then when he took power, it became clear that he'd never had any intention of following in Franco's footsteps, since he started introducing democratic reforms almost immediately. After he denounced a 1981 coup attempt made in his name, even the communists started praising him.
To sum up, it was Franco who brought about the restoration of the monarchy, and it was Juan Carlos I's commitment to democratic ideals that brought the restored monarchy legitimacy. These are circumstances that are difficult to replicate, even in Portugal, one of Spain's next-door neighbors.
What I believe is needed is a demise if Salazar eArlier late 50s or very early 1960s. Then someone like Caetano could both take control of country and bring monarchist to throne which eliminates the need for president and someone who constitutionally could sack him. If this is coupled with either a continued estado novo or gradual democracy we could have a continued monarchy in Portugal.
PS current Portuguese claimant to throne served in the colonial wars but was sent home for criticizing its conduct.
He was a helicopter pilot and like any Portuguese served his country, did not go away to avoid service. Like many he was critical of the regime but his stature and ability for his words to be transmitted was sent away. Not like they could put him in prison or reprimand.I heard about that, but wasn't sure, as to what he viewed was wrong in how the war was conducted.