UA at SEA might also be able to do this for you. In December I flew SEA-IAD-ATL-IAD-SEA on UA but booked as a US code share (and I'll never do it again)..but when I arrived at SEA to check in the very helpful agent at the counter not only confirmed my upgrade for that day SEA-IAD-ATL but they confirmed me in first ATL-IAD and waitlisted me IAD-SEA (ATL-IAD-SEA was a few days later)...something that I did not believe was possible. That leads me to believe if you get a good agent at any airport they can override this silly day-of-departure only rule on code shares and get you confirmed or at least on the wait list. I would certainly ask when your'e at SEA, but as the other poster said, you can also try at ORD.