The Air China phone agent did not have the strongest grasp of the English language but was able to select my window seat (2L) on the right side of the airplane.
A few weeks after I booked my ticket with United, I called Air China’s North American call center (located in Los Angeles) to select my seat. Like I said earlier, Air China releases a ton of award seats to their partner airlines and I had no problem finding award space on the dates I needed.