Preheat oven to 350F with a rack in the center. place bead cubes in a shallow 2 1/2 quart baking dish; set aside. Place raisins in a small bowl. Add zests and juices; let stand 30 minutes.
Place eggs in a large bowl. In another large bowl, combine heavy cream, 1 cup sugar, vanilla, and nutmeg. Pour the cream mixture into eggs, whisking constantly until fully combined. Add the raisin mixture, and stir to combine. Pour over bread, making sure all the bread is thoroughly saturated. Sprinkle with remaining sugar.
Brush a sheet of parchment paper with butter. Place buttered-side down, over the bread. Place baking dish in a larger roasting pan. Place in oven, and add enough water to the larger pan to come halfway up sides of baking dish. Bake 45 minutes. Uncover and bake 15 minutes more. Remove from oven and let stand 20 minutes before serving. Top with a dollop of whipped cream, if desired.