I don't know if they allow this, but I seem to remember that they don't. I flew out of there twice last summer (ORD flight both times) and my impression was that the counter staff was generally unwilling to do anything except move the current flight through as fast as possible. The line is long and they have a lot of people to process in a short time, so they most likely won't want to deal with you early. I did see several people turned away and told to come back later, though I can't tell you why they were turned away.