buschoi

I flew this exact route (SEA-YVR on AS893 and YVR-HKG on CX889) a few months back.
AS in SEA was able to print the boarding pass for CX.
AS in SEA was able to check the bags all the way to HKG. However, AS in SEA said because of the arrival time in YVR, the transit area would be closed. I would have to pick up my baggage in YVR, go through immigration and customs to enter Canada, go upstairs to drop off the bags at CX's check-in counter, and then go through immigration and customs again to exit Canada.

I got to YVR, waited for my baggages that never showed up, asked the AS ground crew there and was told no need to pick up baggages. Just go through immigration and customs to enter Canada (nothing to declare of course), go upstairs, and go through immigration and customs to exit Canada. After going through the exit immigration at YVR, there were Canadian customs officers asking all international passengers how much cash they had to make sure they were not over CAD10K. When the Canadian customs also asked me how long I was in Canada, I literally told them 15 minutes. Their eyes widened until I told them I was there in transit, and they let me go.

Coming back on HKG-YVR-SEA is a bit different. CX could not print AS's boarding pass. The TSA at the YVR pre-clearance area would not allow anyone to go through without a boarding pass. So at YVR's transit area (right before TSA and passport desk), I had to use a phone on the wall to call an AS agent to come over, and she would print a boarding pass right there. I have done this several times, and each time I have to wait between 10-20 minutes for the AS agent to show up. The AS agent said she would have to go through security to get there, and that took some time.

Make sure you print a copy of the Canadian ETA. It is supposed to be electronically tied to my passport, but for some mysterious reasons, the system was never able to find it. AS in SEA can't find it, and they were happy as soon as I show them my paper copy. CX in HKG has to call Canada every single time to verify and get an exception code when I check-in. CX in HKG said they had to call Canada 20-30 times every day to get that exception code. Even the Canadian immigration officer at the transit area did not see it when I was doing a transfer for SEA-YVR-HKG on another day flight. I had to show him my paper copy before he would let me go while he was scratching his head. He said the system has had problems. Not sure if it has been fixed and gotten better now. I have avoided transiting in YVR now for the ETA problem and have been using SFO and LAX instead.