Sprinkle roast with salt, thyme, garlic powder and ground pepper. Cover and refrigerate at least 8 hours or up to 24 hours. Meanwhile, in a saucepan, saute onion in 1 tablespoon oil until tender, about 5 minutes. Add wine, thyme, garlic, peppercorns and cloves. Simmer until reduced to 3/4 cup. Cool; strain, discarding solids, and refrigerate., Remove roast from the refrigerator 1 hour before grilling. Prepare grill for indirect heat, using a drip pan. Add wood chips according to manufacturer's directions., Pat roast dry with paper towels. Brush with remaining 1 tablespoon oil; place over drip pan. Grill, covered, over medium-low indirect heat, brushing with mop sauce every 20 minutes, until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°), 1-1/2 to 2 hours. Let stand 15 minutes before slicing., If desired, in a small bowl, stir together butter, horseradish and thyme. Serve on top of roast.