In a bowl, whisk together soy sauce and next 5 ingredients.
While whisking, add 2 tablespoons of the oil in a thin stream until emulsified.
Divide mixture in half.
Place bluebill breasts in a non-reactive container and pour half of marinade mixture over.
Cover and refrigerate for 2 - 4 hours, turning occasionally. Remove from marinade. Discard marinade.
Whisk cornstarch mixture into reserved marinade.
Heat remaining 2 tablespoons oil in a large skillet or wok over high heat.
Add duck breasts and stir-fry for 2 minutes.
Add peppers and cook for 2 minutes.
Add snow peas, plums and reserved marinade and bring to a boil.
Remove duck and arrange over mounded warm rice.
Continue heating sauce until thickened.
Stir in green onions and spoon sauce mixture over duck.