Tear the sandwich bread into small pieces, then toss with the milk and soy sauce in a large bowl. Let stand 10 minutes.
Meanwhile, mince the white scallions, pork, garlic, ginger, water chestnuts, egg, cilantro, 1 teaspoon sesame oil, 1 1/4 teaspoon salt and 1/2 teaspoon pepper to a bowl with the bread; mix until combined. Gently form into 16 golf ball-size meatballs. Put meatballs on a baking sheet, cover with plastic wrap and refrigerate 1 to 4 hours.
Stir the hoisin sauce, mayonnaise, 1 1/2 tablespoons lime juice, the chili-garlic sauce, and the remaining 1/2 teaspoon sesame oil in a medium bowl. Cover and refrigerate until ready to serve.
Heat about 1 1/2 inchs peanut oil in a large, wide saucepan over medium heat until a deep-fry thermometer registers 350 degrees. Add the meatballs in batches and cook, turning occasionally, until browned and cooked through, 8 to 10 minutes. Transfer to a paper towel-lined plate to drain.
Drain the green scallions and toss with bean sprouts, the remaining 1 tablespoon lime juice, and a pinch of salt in a bowl. Spread hoisin mayonnaise on the inside of the baguette pieces, then fill with the meatballs and top with the scallion-sprout mixture.