I'm actually surprised it's always so empty, Shenzhen Airlines operates a fair number of regional international flights now, though not sure if they are the same time as yours. I'd assume the UA codeshares are to Japan?

I haven't done it yet, but have been tempted to try them to HKG due to them sometimes being half of what ANA charges from Tokyo, also they are almost always available for a C class redemption (TYO-SZX) and China is North Asia where UA charges more mileage for redemption to HKG (South Asia). I have traveled into SZX on OZ a few times in the past, but never departed there, always departed HKG or CAN.