Actually, you may be able to get the exit row ahead of time.

I happened to be talking to a coworker yesterday while he was booking an April ticket to MSY and noticed that the exit row seats were available to be selected. He got the standard warning (age, understand instructions, blah blah) to accept before being assigned the seats.

I then called JetBlue and got my seats in May to LAS changed to the exit row. She said that you can now get them when booking (but there's still no way no change seats online after the ticket is purchased).