I admit that I have never read Il Fu Mattia Pascal, but I can try to answer you even not knowing the context within the narration of the book. The name "Santa Maria Liberale" could be a reference to the true Venetian church "Cathedral of Santa Maria Assunta and San Liberale" whose church is dedicated to both Santa Maria Assunta and San Liberale.
San Liberale is the patron saint of the city of Treviso and Castelfranco Veneto (the place where the cathedral is located).
Brief mention of San Liberale: he was from a rich pagan family, he wanted to enlist as a soldier from an early age, converted to Christianity with the aim of helping the poor and praying.
However, I don't think the use of the word "Liberal" has a political background in this case. Also because there is little relevant information on liberal Christianity (wiki)
The names they take are clearly not references to the word itself. Most of them have their real names: "San Martino", "San Andrea", "Santa Lucia". Others instead have the addition from the place they come from: "Sant'Antonio da Padova" (from Padua), "Santa Teresa d’Avila" (from Avila), "San Francesco d’Assisi" (from Assisi). Others instead have canonical additions based on what they did during their life, for example: "San Gabriele dell'Addolorata", who does not come from Addolorata, but is given by the fact that he has indulged the devotion to the Madonna Addolorata rooted in him since childhood, inspired by a statuette of the Pietà that his mother kept at home.