I jumpseated out of SIN and HKG on UAL a few years ago and as soon as I mentioned OMC the agents were all aware and knew what to do. You have to go to the ticket counter (usually the additional/special services desk) to get listed and pay the departure taxes. The taxes can vary widely on location. Out of SIN and HKG they were not too bad, in the area of $20-$30 USD. But keep in mind that was a few years back. One of the things I noticed is both SIN and HKG keep you at the ticket counter until the gate agents report space available. So you have to make the mad rush through security and out to the gate. They will literally close the door right behind you as you step on to the aircraft. It makes talking to the captain a bit difficult as they are wanting to push as soon as you get on the aircraft.
When I have come out of Europe on UAL I have never had the issue of being held at the ticket counter until the last minute, they always give me a standby boarding pass.