I did BKK from IAD via:
PEK - got free night in Beijing on Air China. There is no way you will make your flight even if IAD-PEK flight arrives on time. Not enough time.
NRT - Did it with UA and ANA. About 2 hours connection time in Narita.
HKG - with AC via YYZ. Loooong flight, but when you get to HKG you only have 2 more hours to go
LHR - This flight is actually the fastest one, 6 hours to LHR then 10.5 hours to BKK