I've found myself on CO a number of times this year and have also been able to select priority seats in advance for some flights--but only when booking via Expedia. For flights I've booked on continental.com or Orbitz, I've either been unable to select priority seats at all until check-in (continental.com) or kicked back into non-priority seats after requesting priority seats (Orbitz). In addition, at one point while flying a domestic CO itin shortly before an upcoming international itin where I was in the very back of the bus, I asked a friendly TA to see if he could move me to a priority seat on the int'l flights to save me some time in the customs/immigration line. He apologized and said that my request for priority seats wouldn't stick until check-in (at which point I successfully snagged a window in the second row of Y).

In sum, YMMV, although you may find yourself able to secure advance priority seats through Expedia.