“le fils de la Folie et de Pulcinello.” A supposititious Italian dancer, sent to amuse Bettina in the court of the Grand Duke Laurent. Bettina was a servant on a farm, in love with the shepherd Pippo. But when she was taken to court and made a countess, Pippo was forbidden to approach her. Bettina languished, and to amuse her a troop of Italian dancers was sent for, of which Saltarello was the leader. He soon made himself known to Bettina, and married her. Bettina was a “mascotte” (q.v.), but, as the children of mascottes are mascottes also, the prince became reconciled with the promise that he should be allowed to adopt her first child. (La Mascotte.)
Hence a Saltarello is an assumed covert to bring about a forbidden marriage and hoodwink those who forbade it.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Saltarello from Fact Monster: