Process shortbread in a blender or food processor until graham cracker crumb consistency (about 1 1/3 cup crumbs).
Stir together shortbread crumbs, 3 tablespoons sugar, and 2 tablespoons melted butter in a small bowl. Press mixture lightly into a greased 9-inch tart pan.
Bake at 350 degrees for 10-12 minutes or until lightly browned. Set aside.
Combine 1/2 cup sugar, cornstarch, and salt in a medium-size heavy saucepan. Whisk in juice and egg yolds. Cook over medium-high heat, whisking constantly, 10-12 minutes or until mixture thickens and boils. Remove from heat; stir in 3 tablespoons butter and rind.
Pour filling into prepared tart shell. Cover surface of filling directly with plastic wrap. Chill 2 1/2 hours.
Place red grapefruit sections in an 8-inch baking disk. Sprinkle with 2 tablespoons sugar, and chill until ready to assemble. Drain grapefruit before assembly.
Arrange segments, with outer part of segments facing the edge, around border of tart. Arrange remaining segments around tart, slightly overlapping to cover filling completely. Serve immediately. Chill leftovers.
To make ahead: Prepare crust and filling up to 2 days ahead, but do not top with fruit. Top with grapefruit just before serving.