They'd likely need fifth rights (this is the right one?) - the right to pick-up paxs in HKG and take them to BKK - to make it even feasible to consider. UA has the ability to condense a mutliple planes worth of paxs (to NRT) into one for the final NRT-SIN leg and I think they do this on some other routes as well. With maybe two planes to NRT and two to HKG (at most) per day, AC doesn't have enough to make it viable. I agree that some type of connection changes with TG would make sense, but both AC and TG would need to compromise.