Thanks to all for the very helpful replies. Got the bluey (well, actually a goldy) signed with no fuss. Agent was great, knew exactly what to do and she even called up TG to make sure the booking showed up correctly in their system.

I forgot to try it at the in-town checkin, but I have a hunch the contract staff there don't handle UA ticketing issues like this. Next time I'll just make the booking so that I can get to Kowloon on a Saturday morning, despite my intense hatred of Harbour City.