Combine 8 cups cold water and 1/2 cup salt in a 13 x 9-inch glass or ceramic baking dish, stirring until salt dissolves. Add pork to brine; let stand at room temperature 1 hour.
Prepare grill to medium-high heat.
Drain pork; pat dry. Brush pork with oil; sprinkle with 1/4 teaspoon pepper. Place pork on grill rack; grill 15 minutes or until a thermometer registers 155° (slightly pink), turning pork occasionally. Remove pork from grill; let stand 5 minutes. Cut across the grain into 1/2-inch-thick slices.
Combine remaining 1/4 teaspoon pepper, plums, and next 7 ingredients (through chile); toss gently to combine. Serve relish with pork. Garnish with parsley sprigs, if desired.