I have used this trick with great success on UA. UA's 763s are in a 2-3-2 config. If the middle row is open I occupy the middle seat and expand as much as possible to fill the other two seats. The trick is to fill the space as much as possible. Stick your elbows way out into the other seat, have a cough during boarding while you are sitting there, deploy all the tricks you can get your hands on.

It sounds like you were in 4 abreast seating (Y on a 744 maybe, KF do you ride in Y?????) and the original seating was -+--. Well this is what I would do. 1) first I would sit in the blocked seat so it would be +---. 2) I would put as much crud as I could into the aisle seat. 3) If someone goes to sit down in your original seat I would get up, take the aisle seat, and offer the new person the middle with the warning that you have a cold. (punctuate that with a nice wet sneeze).

That should be sufficient to keep the empty seat.