Combine catsup, 2 cups vinegar, lemon juice , 1 tablespoon hot sauce, Worcestershire sauce, olive oil, mustard, and garlic salt in a small Dutch oven; stir well. Cook, uncovered, over low heat 1 hour, stirring occasionally.
Combine 1/2 cup cider vinegar and 3/4 teaspoon hot sauce; stir well. Set aside.
Place roast 6 to 8 inches from slow coals. Insert meat thermometer, being careful not to touch bone or fat. Grill 6 to 7 hours or until meat thermometer registers 170°, turning frequently and basting frequently with vinegar-hot sauce mixture. Baste meat with barbecue sauce during last hour of grilling.
Slice meat thinly , and toss with enough barbecue sauce to coat well. Remaining barbecue sauce may be kept refrigerated for other uses.