Combine chicken and lemon zest and juice in a large bowl; toss to coat. Sprinkle with salt and 4 tbsp. seasoning and toss again to evenly coat. Cover and let chill at least 2 hours and up to 12 hours.
Heat 1 Tbsp. oil in a medium saucepan over medium-high. Cook onion and garlic, stirring occasionally, until tender, about 3 minutes. Add brown sugar; cook, stirring constantly, until sugar turns a shade darker, about 2 minutes. Add remaining 1 Tbsp. seasoning; cook, stirring, until fragrant, about 30 seconds. Add ketchup; cook, stirring, until slightly darkened in color, about 2 minutes. Stir in vinegar, molasses, mustard, hot sauce, and Worcestershire sauce. Bring to a boil over high heat; cook, stirring, 2 minutes. Let cool 5 minutes. Transfer to a blender and purée until smooth. Set sauce aside.
Prepare a grill for medium-high indirect heat(for a charcoal grill, bank coals on 1 side; for a gas grill, leave 1–2 burners off). Lightly oil grate. Grill chicken over direct heat, turning every minute, until browned on all sides, about 5 minutes. Move chicken over to indirect heat, cover, positioning vent over chicken if your grill has one, and grill, turning every 5 minutes or so, until an instant-read thermometer inserted into the thickest part of thighs registers 140°–145°, 18–25 minutes. Uncover grill and continue to grill, basting with reserved sauce and turning occasionally, until thermometer registers 165°, about 10 minutes longer.
