In a small bowl, mix 2 1/2 teaspoons salt, 1 1/2 teaspoons black pepper and the cayenne. Set aside 1/2 teaspoon of the spice mixture. In a large bowl, toss the brisket with the remaining spice mixture.
Preheat the oven to 300 degrees . In a large, heavy skillet, heat the olive oil over medium heat. Working in batches, add the brisket and cook until browned, 8 to 10 minutes. Transfer to a large roasting pan.
Add the beer to the skillet, lower the heat and simmer for 2 minutes, scraping up the browned bits. Pour the beer mixture into the roasting pan, cover and cook in the oven until the meat cuts easily with a fork, 3 1/2 to 4 hours. Let cool.
Shred the beef and transfer to a large saucepan. Pour the remaining pan juices into a bowl and skim off the fat. Add 2 cups of the juices to the shredded beef, cover and keep warm over low heat.
In a small saucepan, heat the ketchup, vinegar, brown sugar, mustard and the reserved 1/2 teaspoon spice mixture over medium heat. Stir 1 cup of the ketchup mixture into the beef, setting aside the rest for serving. Cover the beef and cook, stirring occasionally, until heated through.
Split the baguettes and hollow out slightly. Mound the beef into the bread bottoms; cover with the bread tops. Cut each into 4 pieces and serve with the reserved sauce.