To prepare custard, combine first 3 ingredients in a medium saucepan, stirring well with a whisk. Stir in 1 1/2 cups milk; bring to a boil over medium heat, stirring constantly. Cook 1 minute, stirring constantly; remove from heat. Combine remaining 1/4 cup milk and egg in a small bowl. Stir one-fourth of hot milk mixture into egg mixture. Add egg mixture to remaining milk mixture in pan, stirring well. Cook over medium heat 1 minute or until thick, stirring constantly. Remove from heat; stir in pumpkin and vanilla. Pour mixture into a bowl; cover surface with plastic wrap. Chill completely.
To prepare pears, combine lemon juice and pears. Melt 1 tablespoon butter in a large nonstick skillet over medium-high heat. Add pear mixture; cook 4 minutes. Drizzle 3 tablespoons liqueur over pears; cover and simmer 5 minutes or until tender. Remove pears from pan with a slotted spoon, reserving liquid in pan. Add syrup to pan; bring to a boil. Cook 1 minute, stirring frequently. Pour syrup mixture into a small bowl; stir in remaining 3 tablespoons liqueur. Cover and chill.
Line the bottom of a 2-quart trifle bowl with half of cake slices (cut slices to fit bowl, if necessary); drizzle with half of liqueur mixture. Top with half of the pear mixture and any accumulated juices. Spoon half of chilled custard over pears. Repeat procedure with remaining cake, liqueur mixture, pear mixture, and custard. Spread 1 cup whipped topping over custard, and sprinkle with 1/4 teaspoon cinnamon. Cover and chill for 3 hours.