I did something simular once. I was on a trans-con 763 3-class. I was confirmed in C class and was the last one on the plane. I originally had a window and there was already a pax in the aisle so took 5C (my favorite seat). I began to read and noticed I was being ignored for the first hour of the flight, I got up and went to galley and told them I had moved. The FA's just laughed, and wondered who I was. Turns out they counted a deadheading FA as a pax and thought I had sneaked in and were arguing whom was going to confront me. So I find it hard that some can do this from LAX-SYD without being caught.