Soak 6 to 12 thin wood skewers (8 to 10 in. long) in water at least 15 minutes.
Meanwhile, trim off and discard tough tops and root ends of lemon grass. Remove and discard tough outer layers of stalks until you reach tender inner part. Cut tender part into 1/4-inch pieces.
In a blender, combine lemon grass, garlic, onion, oyster sauce, hoisin, soy, honey, and pepper; whirl until finely ground and soupy.
Trim and discard fat from pork. Slice meat across grain as thin as possible, then slice into about 1-inch-wide strips. Coat pork slices with lemon grass mixture.
Weave wood skewers in and out of meat strips, down the center, at 1-inch intervals. Loosely push meat together so it bunches up, filling skewers equally.
In a 5- to 6-quart pan over high heat, bring about 3 quarts water to a boil. Add rice noodles and cook until they are barely tender to bite, 3 to 5 minutes. Drain, then immerse the hot noodles in cold water. Let stand until cool, then drain.
Divide lettuce, bean sprouts, and mint and cilantro sprigs among 6 wide soup bowls. Mound equal portions of noodles in the bowls.
Place pork on a barbecue grill over a solid bed of hot coals or high heat on a gas grill (you can hold your hand at grill level only 2 to 3 seconds); cover gas grill. Cook pork, turning to brown the meat on all sides, 6 to 8 minutes total.
Place skewers of meat on noodles in bowls. Sprinkle with peanuts. Push meat off skewers and eat with condiments in bowl, adding sweet and sour fish sauce to taste.