So your current routing is HKG-SEA-JFK? You used to be able to SDC from a regular F routing to a D1 routing, but they have (officially) removed this option. You might find an agent willing to help you out here.

One other thing to consider, though, is that if you are on a GUC, you will probably be at the top of the upgrade list, so they might let you SDC to the flight in Y and clear the normal upgrade list.

And yes, you can only request it after all international segments on your itinerary are complete. If you have a return to HGK on this ticket it won't work.