Combine the mango pulp, agave syrup and three-quarters of the orange zest in a small saucepan. Using tongs, lightly char each bay leaf directly over the flame of a gas burner on high heat, 3 to 4 seconds. Add 2 of the toasted bay leaves to the mango mixture and bring to a boil, then remove from the heat and let cool completely.
Pour the mango mixture into a small pitcher, discarding the soaked bay leaves. Stir in the mezcal and orange and lemon juices. Add ice and stir until well chilled, at least 15 seconds.
Combine the salt and remaining orange zest on a plate. Dip the rims of 4 glasses in water, then dip in the orange salt to coat.
Pour 2 tablespoons grenadine into each prepared glass. Fill the glasses with ice, then pour in the cocktail. Top with seltzer and garnish with the remaining toasted bay leaves.