This happened to me a few weeks back on a LHR-BKK-MDL. I asked as politely as I could that UA contact TG and ask them if it would be possible to open up space on some alternative TG services. The UA agent couldn't have been any ruder. "I definitely can NOT ask them." You'd have thought I was asking for her firstborn child ...

I also contacted my TG reservations office and asked them who could help. The guy said, "I have no idea." At least he was honest.