Prepare the pork: Combine the ale, five-spice powder, cayenne and salt and black pepper to taste in a large resealable plastic bag. Reserve 1/4 cup marinade for the slaw. Add the pork to the bag and mix well. Refrigerate 2 to 6 hours.
Make the glaze: Heat the olive oil in a saucepan over medium heat. Add the onion and garlic and cook until golden, about 2 minutes. Add 1 tablespoon each stout and brown sugar and cook 4 minutes. Add all but 2 tablespoons of the remaining stout, the honey, the remaining 3/4 cup plus 3 tablespoons brown sugar and the ginger. Bring to a boil over medium-high heat, stirring, then lower the heat to medium and simmer until reduced by half, 15 to 20 minutes. Whisk the remaining 2 tablespoons stout and the cornstarch in a bowl; slowly stir into the glaze and boil 2 minutes. Remove from the heat and let cool.
Preheat the oven to 375 degrees F. Remove the pork from the marinade and transfer to a baking dish. Bake until a thermometer inserted into the center registers 160 degrees F, 35 to 45 minutes. Transfer to a cutting board and let rest 5 minutes.
Meanwhile, make the slaw: Whisk the soy sauce, honey, garlic and ginger in a large bowl. Whisk in the reserved 1/4 cup marinade. Add the cabbage, bell peppers, carrots and onion and toss. Refrigerate until ready to serve, at least 30 minutes.
Slice the pork and serve with the slaw. Drizzle both the pork and slaw with the glaze.