UA codeshare with AC on SEA-YVR. For cabotage, the operating carrier doesn't matter, the ticketing carrier matters. UA can ticket SEA-YVR-ORD just like AA can ticket US-YVR-JFK.
Actually, it turns out the international segment's what matters. I was thinking about how QF doesn't allow JFK-LAX travel without a TPAC sector on QF, but all that's actually required is an international segment- their rules are more strict than actual DOT cabotage rules.