Grape leaves in brine can be found in the international-foods section of many supermarkets.
Rinse and separate grape leaves. Place in a large roasting pan and cover with boiling water. Let sit until water has cooled somewhat. Rinse in cold water and drain. Pat each leaf dry, clip off stem, and layer between paper towels. Set aside.
In a small saucepan, combine couscous, 1/3 cup boiling water, and cumin. Season with salt and pepper. Cover and let stand until couscous is soft and fluffy when stirred with a fork, about 5 minutes.
Transfer couscous to a medium bowl and add tomato, parsley, 1 tablespoon oil, and lemon juice. Toss together and add salt and pepper; the couscous should be slightly overseasoned.
Lay out 10 grape leaves at a time on a clean work surface. Place a goat-cheese disk 1 inch from each stem end. Mound a heaping teaspoon of couscous over goat cheese. Fold stem end over filling, then fold sides of leaf over. Dab edges with olive oil to help seal. Fold top of leaf over folded sides and dab with oil again. Place on a plate, folded side down, layered between waxed paper. (Leaves can be prepared up to this point in advance and stored, refrigerated, overnight.)
Heat grill or nonstick pan. Brush stuffed leaves with olive oil and place on grill or pan smooth side down. Grill or saute until browned, about 2 minutes. Turn carefully and continue cooking until cheese is soft and filling is heated through, about 2 minutes more. Serve immediately.