If you are talking about a 14-15hr flight I assume it's an international flight.

If there are seats available for selection I'm not sure what they will do. I can say that I did an SDC recently where there were no coach seat assignments available. The phone agent said, as a DM, they would provide a bulkhead seat assignment with the understanding that blablabla if they needed it (handicap, etc.) I could be asked to move.

So I was placed in 5C...but I actually received my upgrade at the gate to the only open FC seat.

Not exactly your situation, but that's the only data point I have for advance bulkhead seat assignments.