Like Madpick said, there's no clear answer on this, and logic is increasingly useless when applied to firearm laws. I agree with his interpretation of the RCW definition of firearm as it applies to stripped lowers (that it doesn't apply to them), but I know others disagree.
Having said that, I asked an attorney about this (after 594 passed, but prior to it going into effect), and he said that when you form a trust, you have to assign it assets, but that it doesn't necessarily have to be monetary (could be anything of value). There was no reason that non-NFA firearms couldn't be those assets of value.
Bottom line, I think that no matter how you do it, I doubt it's ever going to be an issue. That may or may not be good enough for you, so YMMV.