Place plums in a bowl; add boiling water. Let stand for 15 minutes; drain well. Mince enough rosemary to measure 1 tablespoon; stir into plums. Cut a slit along top of roast 2 in. deep, leaving 1 in. at each end uncut. Sprinkle inside of cut with 1/4 teaspoon salt and pepper. Fill with plum mixture. Cover top with overlapping rosemary sprigs.
Tie roast at 1 in. intervals with kitchen string. Sprinkle with the remaining salt and pepper. Place several rosemary sprigs in a shallow roasting pan. Top with roast. Bake, uncovered, at 350° for 1 to 1-1/4 hours or until a thermometer reads 160°. Let stand for 10 minutes before slicing.