My two most recent experiences with Alaska Lounge in SEA yielded different outcomes: - BAEC membership. No AA membership
(i) Nov 2017 - SEA-JFK (AA First) connecting to JFK-LGW (BA Club World) - Access Granted to Alaska Lounge
I asked the AA staff at the check in counter whether we (my wife and I) were entitled to any lounge, he directed us to Alaska Boardroom where we were granted access with no hassles whatsoever. The lady at the entrance of the lounge verified our three boarding passes (SEA-JFK; JFK-LON; LON-MRU) and welcomed us in with a large smile.
(ii) June 2018 - SEA-Dallas (AA First) purely domestic- We were BAEC Silver then. Had 6 hrs layover to kill and entered Alaska Boardroom only to be denied access and told we needed to have AA membership to get entry.
So to recap: if flying AA (First) followed by International OneWorld flight in Business, then I presume it has to be a YES.
If AA only (First), irrespective of OneWorld Status, NO ACCESS.