Heat oven to 325 degrees. Butter an 8x4x2 1/2 inch loaf pan; dust with flour; tap out excess.
Whisk flour, baking powder, salt and nutmeg in a small bowl until blended.
In another small bowl, mash bananas with lemon zest and juice; stir in rum.
Microwave chocolate and 2 T butter, stirring every 10 seconds, until melted and blended.
Beat remaining butter with mixer on medium speed 3 minutes or until creamy. Add sugars; beat 2-3 minutes until light and smooth.
Add eggs, 1 at a time. Beat in vanilla (batter will look curdled).
Reduce mixer speed to low and beat in 1/2 the flour mixture just until blended. Beat in milk until combined, then remaining flour mixture. Scrape down bowl; beat in banana mixture (batter will look lumpy).
Stir a little less than 1/2 the batter into melted chocolate to blend. Drop alternating spoonfuls of both batters into prepared pan. Using a table knife, swirl batters together, taking care not to overdo it.
Bake 1 hour and 20 to 30 minutes (check cake after 45 minutes and cover loosely with foil if it's getting too brown), until a wooden pick inserted in center comes out with moist crumbs attached.
Cool in pan on a wire rack 15 minutes before inverting on rack, turning right side up and cooling completely.