For the French toast: Butter a 9-by-13-inch baking dish. Arrange the croissant halves in the baking dish. In a large bowl, whisk together the eggs, milk, half-and-half, nutmeg, vanilla, Grand Marnier, orange zest, 1/4 cup brown sugar, white sugar and salt. Pour this mixture over the croissants. Cover the baking dish and chill until all of the liquid is absorbed, at least 2 hours.
For the topping: Preheat the oven to 350 degrees F. In a shallow baking pan, spread the pecans evenly and toast until fragrant, about 4 to 5 minutes. Remove from the oven and toss pecans with the 1 teaspoon butter and salt.
Increase the oven temperature to 400 degrees F. Sprinkle the blueberries evenly over the croissant mixture. Melt 1/4 cup butter with 1/4 cup brown sugar, stirring until the butter is melted. Drizzle this mixture over the baking dish and bake for 20 inutes, or until the liquid from the blueberries is bubbling.
For the blueberry syrup: In a small saucepan, cook blueberries and maple syrup over moderate heat until berries have burst, about 3 minutes. Pour syrup through a sieve; stir in the lemon juice.
Sprinkle the croissant French toast with toasted pecans. Serve with the syrup.