Line loaf pan with plastic wrap, leaving an overhang at each end. Purée melon with 1/3 cup sugar, grappa, grenadine (if using), and salt in a blender until smooth.
Transfer to large metal bowl and stir in yolks. Set bowl over a saucepan of simmering water and beat with mixer until pale, thick, and tripled in volume and registers 170°F on an instant-read thermometer, about 8 minutes.
Remove bowl from saucepan and set in a large ice bath. Continue to beat until cold, about 6 minutes.
Beat cream with remaining 2 tablespoons sugar in other large bowl using cleaned beaters until it just holds soft peaks. Fold whipped cream into cantaloupe mixture gently but thoroughly.
Pour mixture into pan and freeze, loosely covered with plastic wrap, until firm, at least 5 hours.Uncover top and invert semifreddo onto a plate.
