Combine 1 1/4 cups low-fat milk and peppercorns in a medium heavy saucepan; bring to a simmer. Remove from heat. Let stand, covered, for 10 minutes. Strain milk through a cheesecloth-lined sieve into a bowl; discard solids.
Place egg yolks in a bowl. Gradually add the hot milk to egg yolks, stirring constantly with a whisk. Return milk mixture to pan. Add the granulated sugar; cook over medium heat until the milk mixture coats a metal spoon (about 3 minutes), stirring constantly. Combine 3/4 cup low-fat milk, vanilla, lime rind, lime juice, and condensed milk in a medium bowl. Gradually add custard, stirring with a whisk. Cover and chill completely.
Pour mixture into freezer can of an ice-cream freezer; freeze according to manufacturer's instructions. Spoon ice cream into a freezer-safe container; cover and freeze 1 hour or until firm.
Combine brown sugar and rum in a small microwave-safe bowl; microwave at high 1 1/2 minutes or until sugar dissolves, stirring after 45 seconds.
Prepare grill or broiler.
Place pineapple on grill rack or broiler pan coated with cooking spray, and cook 2 minutes on each side or until lightly browned, basting occasionally with rum mixture. Place 1 pineapple slice on each of 7 dessert plates; top each serving with 1/2 cup ice cream. Garnish with limeslices, if desired.