Cut 3 of the oranges in half crosswise. Cut ends off remaining orange then cut into 4 crosswise slices. Dip cut sides of orange halves and slices into brown sugar. Reserve remaining brown sugar.
Grill oranges over medium heat 12 minutes or until charred, turning slices occasionally. Reserve slices for garnish.
Place orange halves, orange juice, allspice and reserved brown sugar in large saucepan. Bring to boil. Reduce heat to low; simmer 10 minutes. Remove from heat. Cool completely. Remove orange halves and squeeze juice back into saucepan. Mix well. Strain juice. Refrigerate until well chilled.
Place juice mixture, coconut milk, rum, vanilla and 1 cup of the ice in blender container; cover. Blend on high speed until smooth. Add additional ice as desired. Pour into beverage glasses. Garnish with a charred orange slice and a maraschino cherry.