Butter a 13 by 9-inch baking dish. Cut 20 (1-inch) slices from baguette and arrange in 1 layer in the baking dish. In a large bowl whisk together eggs, milk, nutmeg, vanilla and 3/4 cup brown sugar and pour evenly over bread. Chill mixture, covered, until all liquid is absorbed by bread, at least 8 hours, and up to 1 day.
Preheat oven to 350 degrees.
In a shallow baking pan, spread pecans evenly and toast in middle of oven until fragrant, about 8 minutes. Toss pecans in pan with 1 teaspoon butter and salt.
Increase temperature to 400 degrees.
Sprinkle pecans and blueberries evenly over bread mixture. Cut 1/2 stick butter into pieces and in a small saucepan heat with remaining 1/4 cup brown sugar, stirring until butter is melted. Drizzle butter mixture over bread and bake for 20 minutes, or until any liquid from the blueberries is bubbling.
Meanwhile, in a small saucepan, cook blueberries and maple syrup over moderate heat until berries have burst, about 3 minutes. Pour syrup through a sieve into a heatproof pitcher, pressing on solids, and stir in lemon juice. Syrup may be made 1 day ahead and chilled, covered. Reheat syrup before serving. Serve French toast with syrup.