Place pork shoulder, onion, ginger and garlic in bottom of slow cooker. In a bowl, whisk stock, coconut milk, lime juice, curry powder, sugar, 1/2 tsp of the salt and the red pepper flakes. Pour over pork. Cover and cook on HIGH for 6 hours or LOW for 8 hours.
Using tongs, carefully remove pork to a cutting board and shred. Stir remaining 1/2 tsp salt into liquid and return shredded pork to slow cooker. Meanwhile, prepare rice noodles per package directions.
Divide rice noodles among 8 bowls, then ladle some pork and broth on top. Garnish with scallions and chow mein noodles, if using.