I've gone US to CO in IRROPS, and it was not pleasant. The PHL US agent had to pull out the Rolodex for the CO number. It was bad. She then thought to call the UA desk to see if they could do it. No joy. She then called UA back for the CO number, and they were able to accept the ticket.

This was for legitimate IRROPS on the US side and it took about half an hour. Whether they could release it for your situation, and in a timely manner, is questionable.

Needless to say, I wish you the best of luck, as I hate being sentenced to fly US.