To Prepare the Rub and Marinate the Pork:
Combine brown sugar, five-spice powder, salt, and pepper in a small bowl. If there is a very thick section of fat on the pork, slice away some of it, leaving a moderate layer. Rub spice mixture into pork shoulder until well coated. Place pork in a medium roasting pan, cover with aluminum foil and refrigerate overnight.
To Braise the Pork:
Combine chicken broth, fish sauce, lime juice, garlic, shallots, and sriracha in a large measuring cup. Set aside. Remove pork from refrigerator and set aside at room temperature for 30 minutes.
Preheat oven to 250°F. Uncover pork and pour braising liquid into roasting pan. Re-cover with foil and roast until an instant-read thermometer reaches 180°F, about 6 to 8 hours, depending on thickness of shoulder. Remove pork from braising liquid. Pour liquid in a fat separator, or skim excess fat from surface of liquid and discard.
To Prepare the Sandwiches:
Shred pork with two forks. Pile meat inside a baguette. Top with jalapeño, cilantro, cucumbers, and a drizzle of braising liquid.