During the recession it was easier. Around that time I was about 80% on Qantas trans-Pacific op-ups over a year including one flight with a Y->J->F op-up. I'm not sure that seat availability is much worse now than it was before say 2006, but a lot of factors have changed (route map, domestic booking class, stopover rules, number of J seats since premium economy was introduced, elimination of F on most sectors, etc.)