I've also done the jumpseat thing BKK-NRT-PDX on NW a few years ago. I didn't wear my uniform, and the agents in both BKK and NRT were knowledgeable and helpful. It helps to use their lingo though (ie "XCM" on NWA, "ACM" on UA) as I've found that the generic term "jumpseating" only confuses foreign agents because then they think you want to actually ride the jumpseat - which of course is not allowed.

Good luck! Thailand is great!