1. Cook chiles in a skillet over high heat 5 minutes or until fragrant, turning often. Remove stems and seeds from chiles. Place chiles in a large bowl; add 4 cups boiling water, and let stand 20 minutes. Drain.
2. Process chiles, vinegar, next 8 ingredients, and 3/4 tsp. salt in a blender or food processor until smooth, stopping to scrape down sides as needed.
3. Sprinkle pepper and remaining 2 1/4 tsp. salt over brisket. Place brisket in an extra-large zip-top plastic freezer bag or a large shallow dish. Pour chile mixture over brisket; rub brisket with chile mixture. Seal or cover, and chill 2 to 24 hours.
4. Preheat oven to 350°. Remove brisket from marinade, discarding marinade. Wrap brisket with heavy-duty aluminum foil, and place in a jelly-roll pan. Bake 3 hours or until a meat thermometer inserted into thickest portion registers 195° and brisket is very tender. Remove from oven, and let stand 10 minutes. Cut brisket across the grain into thin slices. Squeeze juice from limes over brisket before serving. Garnish, if desired.
Note: Guajillo chiles and Mexican oregano may be found on the spice aisle of specialty grocery stores or in Mexican markets.
*Dried oregano may be substituted for Mexican oregano.