1. In a bowl, mix flour with cornmeal, baking powder, nutmeg, and salt.
2. In another bowl, with a mixer on high speed, beat butter and sugar until well blended. Add eggs one at a time, beating well after each addition. Beat in pumpkin, rum, lemon peel, and vanilla. Stir in flour mixture until well incorporated.
3. Scrape batter into a buttered 8- to 9-cup bundt pan and set in a 12- by 17-inch baking pan. Place on bottom rack of a 350° regular or convection oven. Carefully pour boiling water around bundt pan almost to the level of pudding. Cover entire baking pan tightly with foil.
4. Bake until the pudding feels firm to the touch and a wooden skewer inserted into the center comes out clean, 1 hour to 1 hour 10 minutes. Let cool 10 minutes, then invert over a plate to unmold. Serve warm, or let cool completely and cover loosely until serving. Reheat in a 350° regular or convection oven until warm, about 10 minutes. To serve, slice into wedges.