You don't say what class of service you're looking at booking. If you're going Y, there isn't much better than E+ on UA if you're elligible. So, BOS-ORD/SFO-HKG-SGN might not be a bad choice, then onto HAN with a seperate (paid) ticket. This gives you more flexibility

The only *A carriers to fly direct to HAN are TG from BKK, SQ from SIN & OZ from ICN. BOS-LAX-SIN-HAN might not be bad in Y either, since you'll get SQ Executive Economy on LAX-SIN.

If you're going in F or C, then most *A carriers are better than UA, but none fly to BOS & HAN, so you'll need to mix. UA to an Intl gateway, then a *A carrier transpac and then over to HAN.