Toss together watercress, spinach, scallion greens, parsley, celery, and 1 tablespoon plus 1 teaspoon bread crumbs in a bowl. Melt butter in a 10-inch heavy skillet over moderate heat, then add watercress mixture and cook, stirring, until spinach is wilted, 1 to 2 minutes. Stir in Pernod, cayenne, and salt and pepper to taste, then transfer mixture to a bowl and chill, covered, until cold, about 1 hour.
Put oven rack in middle position and preheat oven to 450°F.
While watercress mixture chills, cook bacon in cleaned skillet over moderate heat, turning, until crisp, then drain on paper towels and finely crumble.
Spread 5 cups kosher salt in a large shallow baking pan (1 inch deep) and nestle oysters (in shells) in it. Spoon watercress mixture evenly over oysters, then top with bacon and sprinkle with remaining tablespoon plus 2 teaspoons bread crumbs. Bake oysters until edges of oysters begin to curl and bread crumbs are golden, about 10 minutes.
Serve warm oysters in shells, nestled in kosher salt (about 5 cups), on a platter.