Preheat oven to 300 degrees. Bake chiles until fragrant and softened, about 5 minutes. Let cool for 2 minutes. Stem, seed, and break into small pieces.
Combine sugar and water in a small saucepan, and set over high heat. Cook, without stirring, until mixture begins to turn gold around edges of pan. Continue cooking, swirling pan, until mixture is light gold. (Do not let caramel darken, or it will result in a bitter marinade.) Immediately add garlic and cook, stirring, for 30 seconds.
Remove from heat, and stir in chiles. Stir in orange juice and vinegar, set over medium heat, and cook, stirring constantly, until syrupy, about 2 minutes. Use marinade immediately.
If desired, turn marinade into a finishing sauce: After removing meat, pour marinade into a nonreactive saucepan, and boil for 3 minutes. Brush on cooked meat just before serving.
For a milder marinade, use ancho chiles -- dried poblanos -- instead of the pasillas. This marinade works best with 1-inch-thick bone-in pork chops. Marinate the pork overnight, and then grill over medium-high heat to desired doneness.