BA has been a solid indicator of LAN availability using AS miles and I have noticed a lack of two seat availability as well. In this time when most airlines are reducing award inventory, depending on you location, it might be prudent to monitor space using AS miles on American, and even AeroMexico.

Beware that it is best to do as you are with BA on LAN-check from the US gateway to EZE or SCL, and then check for seats from your origin to the gateway.
Alaska allows stopovers at the gateways, or you may elect to buy a ticket to the gateway if space on AS is not available.