Not exactly the same thing but one time (as an EXP) I was upgraded into F at the window on flight departing MIA for somewhere in the northeast (may have been BOS or DC, can't remember). I was traveling with a non-status companion and once we arrived at the AC I inquired as to whether the flight had open seats in F or not. The AAgent said that there were in fact 9 open seats. I thanked her, went to the gate, and asked if I could upgrade my companion. The GA was able to confirm her into F instantly. When I verified that the number of stickers required should be two he smiled and said, "don't worry about it, its completely empty."

So with that in mind, I'd never say never. However it's very unlikely that you'll be able to talk your way into a complimentary upgrade. Plan to sit in Y, or pay for it.