I have a complicated F award booked with UA miles (VTE-BKK-HKG-ICN-LAX-SFO), and the HKG-ICN sector on OZ was scheduled as A330 2-class when I first booked it. Just checked yesterday and now it is 74E! I called in to get a seat assignment, and at first I didn't realize this was a 3-class plane being sold as 2-class, so I requested an upper deck J seat. The friendly phone agent said I should just select a main deck seat since it's a single seat next to the window. I was really confused, and then I quickly realized she was suggesting I select one of the F seats at the nose of the plane! I quickly agreed and selected 2K
Very happy for the semi-upgrade, and hopefully they don't swap equipment between now and March 2012! So yes, you can definitely select an F seat even if you are supposed to fly in J. Though I'm not sure if it is because I'm actually on an F award flight or not...