For the ribs: Prepare a grill for direct and indirect heat: For gas grills (with 3 or more burners), turn all the burners to medium-high heat; after about 15 minutes, turn off one of the side burners and turn the remaining burners down to medium. For charcoal grills, bank one chimney starter-full of lit and ashed-over charcoal briquettes to one side of the grill. Set up a drip pan on the other side to avoid flare-ups. (Be sure to consult the grill manufacturer's guide for best results.) If your grill has a thermometer, aim to keep the temperature at about 350 degrees F throughout grilling.
Insert a butter knife between the membrane and bone on the back of the rack of ribs. Grab the membrane with a paper towel to grip it, then pull off the membrane and discard. Sprinkle both sides of the ribs with the brown sugar and seasoning salt and rub it in. Wrap the ribs tightly in aluminum foil and set on the indirect-heat side of the grill. Cook, turning the package once, until the meat is tender and pulling away from the bone but not totally falling apart, 1 hour 15 minutes to 1 hour 30 minutes. Remove from the grill but leave the grill on.
For the slaw: Meanwhile, whisk together the mayonnaise, lemon juice, seasoning salt and 1/8 teaspoon pepper in a medium bowl. Add the cabbage, carrot, celery, celery leaves and parsley and toss to combine. Season with additional seasoning salt and pepper if needed.
Unwrap the ribs and let cool until you can touch the ribs without burning your fingers. With a knife, slit the skin on the back of the ribs and carefully pull out the bones, keeping the slab in one piece. (Use the knife to help hold down the meat while pulling out the bones.)
Move the boneless slab to the direct-heat side of the grill, rounded-side up, and brush the top with 2 tablespoons of the hot sauce. Grill until the underside begins to crisp, about 2 minutes. Carefully flip the slab and brush the top with the remaining 2 tablespoons hot sauce. Continue to grill the ribs until they're glossy and caramelized, 2 to 3 minutes more.
For the sandwich: Scoop out most of the soft insides of the top of the French loaf to make room for the toppings. Brush the cut sides with oil and sprinkle with seasoning salt and pepper. Grill on the direct-heat side of the grill until the bread has grill marks and is nicely toasted, about 3 minutes.
Place the ribs on the bottom of the loaf, brush with additional hot sauce, if desired, and top with the slaw, onions and blue cheese. Top with the top of the loaf. Cut into 4 pieces. Serve with extra hot sauce on the side.