Unfortunately there is no luggage exemption for DL Medallions (any level) on AS.

They only exception, and there it does not matter if you are Medallion or not is connecting to an intl. flight. Mind you only on the outbound. I have paid AS fees when AMS-SEA (on DL) got rerouted to AMS-VYR-SEA (KL/AS). Upon re-ceckin in VYR the nice lady behind the counter was not one bit moved by my insistence that this was not my fault. "You can fly, but if you do not pay, your bags stay here."

From AS' website:
Customers making international connections to other airlines within twelve hours of arriving in your connection city, may follow the baggage allowance and weight restrictions of the international airline - provided a ticketing and baggage agreement is in place between Alaska Airlines/Horizon Air and your other carrier. Please contact Alaska Airlines Reservations at 1-800-252-7522 (TTY/TDD Users: 1-800-682-2221) or Horizon Air Reservations at 1-800-547-9308 for more information.