Since BKK is on the other side of the world from BOS here's what I would do.
[Plus I'd rather connect though LHR T4 any day than through LAX, NRT or wherever (and if you have OW status, you'll have access to the Molton Brown spa)].

1- Fly BOS-LHR-BKK on BA
2- Buy BKK-HKT on TG (in C since it's only 15$ more than Y)
3.1- Either buy a HKT-HKG direct on KA
(that's Dragonair; OW can be bought in HKT for about 160$ in economy)
3.2- Or buy a return HKT-BKK on TG and connect to a BKK-HKG flight (possibly on AY for 100$ OW in Y and around 160$ in C)
4- Return HKG-LHR-BOS on BA

(BTW, note that KA and AY don't fly every day on those routes, you'll need to check extact dates on thier websites)

Hope this helps.