I think the other thing to note here is that it's often quite difficult to find AS award space in the right fare class to add onto an int'l partner award. You should be prepared to find your overwater leg, then hopefully find the connection. If not, book the overwater leg then purchase the connection into SEA. This is what we ended up doing for this coming April SEA-ITM. The outbound had T space to get to SFO. The return didn't, requiring us to purchase the last leg of the journey.