In 2010, My wife and I were upgraded two times out of the 10 individual sectors we took on Economy. Those two times the flight were overbooked but the other 8 times, the flight had a couple of seats open on Y still. We were Silver back then (now Gold).
I have asked the QR staff in the airport what their criteria for upgrading is and its usually dictated by the Y class being overbooked and needing to bump up some people onto J.
I'm not sure if there is actually a systematic way with who QR picks to bump up because my father, who had actually just joined QR PC, on his very first trip using QR got bumped up to J last year. The sector is almost always overbooked though (DOH-MNL).