Purée pineapple and rum in a blender until smooth.
Meanwhile, let ice cream sit at room temperature until soft enough to scoop, 5–10 minutes.
Transfer 1 pint to loaf pan, spreading into an even layer with a spoon.
Dollop half of pineapple mixture over ice cream.
Top with second pint of ice cream, spreading into an even layer with spoon.
Dollop remaining pineapple mixture over.
Tightly wrap with plastic and freeze until cold, at least 2 hours. Serve topped with cherries, if desired.