Originally Posted by eponymous_coward

CX doesn't fly YVR to BOS. Are you confusing BOS with JFK?

You would need to go:

CGK-HKG-YVR CX
YVR-SEA/PDX-BOS AS

You may, in fact, be able to get a stopover in the CX gateway if all that are involved after your stopover are AS flights. I've been successful getting stopovers when only AS segments are involved in the stopover while on CX awards (in fact, I've had nine month stopovers in SEA).

What you won't be able to do is fly CGK-HKG-YVR(stop)-JFK all on CX (or connect at JFK to AA/DL going to BOS, AS only allows one partner per redemption and themselves, and AS doesn't fly JFK-BOS). The only place they will let you take a stopover on CX metal is HKG.