In a medium saucepan, combine the pineapple juice (drained from the can), soy sauce, BBQ sauce, honey and crushed red pepper if using. Cook over medium heat until the sauce simmers. Reduce the heat to medium-low and continue cooking until the sauce thickens slightly, darkens in color and reduces almost by half. Remove from the heat and add the grated ginger. Pour half the sauce into a jar or bowl for serving with the sandwiches. Retain the other half for basting the chicken while grilled. If making ahead, cool the basting sauce to room temperature then add about ⅓ to a large ziplock bag. Add the chicken to the bag and turn to coat. Refrigerate until ready to grill.
Preheat gas grill on high for ten minutes. Turn the heat down to medium. Oil or grease the grill grate using long tongs and clean paper towels soaked in vegetable oil.
Grill the pineapple slices until charred and heated through, turning once. Remove to a clean plate. Grill the chicken thighs until well charred and cooked through, basting with the remaining sauce on both sides. If your grill is large enough, the pineapple and chicken can be cooked at the same time.
To serve, top each bun with lettuce, tomato, chicken, grilled onion rings, pineapple rings and bacon. Pass with extra sauce for drizzling. Serve immediately.