I snagged a F reward LHR to LAX at 22:30 the night before the flight in September. Earlier in the evening I had called the gold line to enquire about availability. The only option at that time was a PE seat. The agent suggested I book that and try for a cash upgrade at checkin. I declined that option (I already had a J seat booked on Aer Lingus). I tried an Hail Mary at 22:30 and low and behold a F award had opened up.
Never stop looking, you never know.