Grease the interior of a 6-quart slow cooker using butter or a nonstick spray to prevent sticking and facilitate easy cleanup.
Scatter half of the bread cubes along the base of the slow cooker. Layer with banana slices and pecans if using, then add the remaining bread cubes to form a second layer.
In a separate large mixing bowl, vigorously whisk together the eggs, whole milk, cream, both types of sugar, vanilla, cinnamon, nutmeg, and salt until fully blended and smooth.
Evenly distribute the custard mixture over the layered bread. Gently press down on the contents to ensure the bread fully absorbs the liquid.
Cover the slow cooker and cook on the LOW setting for 3 to 4 hours, or until the center of the pudding has set and the edges have developed a light golden color.
Shortly before the pudding finishes cooking, begin preparing the sauce. In a small saucepan set over medium heat, melt the butter. Add the brown sugar and stir continuously until it begins to bubble. Carefully stir in the dark rum (or rum extract and water) and vanilla extract, followed by the sliced bananas. Allow to simmer for 2 to 3 minutes, just until the fruit becomes tender.
To serve, ladle warm portions of the bread pudding into dessert bowls and spoon the bananas foster sauce generously over the top. If desired, garnish with a scoop of vanilla ice cream or a dollop of whipped cream.