Over the years I have visited the United’s Bangkok City Ticket Office (CTO) several times for various reasons. In the past, one had to sign a so called “bluey” when upgrading someone else using one of the Systemwide upgrade instruments. Also, issuing an award ticket to someone who doesn’t share the same last name with you, used to require this very same process. I have always received very good service either over the phone or during the visit. They had just received some award from United for their 25 years of service.
The United’s CTO in Bangkok is not actually United at all, but rather their General Sales Agent (GSA) called Air Transport Service. They are, however, able to issue all kind of tickets and help with Mileage Plus related issues as well. You should be able to use paper vouchers there too, although I have never tried.