Quick question for the OP...
Have you tried rerouting thru LAX at all on your DCA trips? AS 5&6 typically have better upgrade odds, plus you might find better seating, for sometimes even cheaper then AS 1,2,3&4?
Otherwise... those last minute purchases do bite... (I typically travel for pleasure, and will purchase my tickets 3,6,9+ months out and get good seating options) and not sure that AS has a viable option for you.
Oh, and you get some extra EQM's on the extra leg, plus an almost guaranteed upgrade SEA-LAX

(and if you time it right, In-N-Out for lunch on the return near LAX

)