I'm not sure, but I'd be somewhat surprised if Washington State cares if the rifle is registered as an SBR. I would guess that as long as it's not actually configured as an SBR while it's in the state, you'd be fine. Consult an NFA experienced attorney, would be my suggestion.
Still happily answering to the call-sign Peetza.
You do not HAVE a soul. You ARE a soul. You HAVE a body.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.