I had the same problem figuring how to get to SEA using only OneWorld flights on a DONE4, next month.
I ended up booking:
BNE-SYD-JFK-SEA-DFW-ANC-DFW-SFO-LHR-SIN-HKG-MEL-PER-ADL-BNE
So it can be done, but it needs some work! Such a pity that the AA/ AS codeshares are not possible on OWE itineraries!
Good luck, and let us know what you end up doing!