OK, not a single person in this thread seems to understand what OP is asking, so let me give some background.

Trusts
Trusts are legal entities used for inheritance. Any kind property can be assigned to a trust. In WA you don't have to "register" this with anyone, a properly formed trust can be assigned any property with paperwork. Trustees of a trust communally control the property in the trust, generally speaking.

NFA
Trusts come in handy with the NFA. The NFA has no provisions for spouses sharing a gun; if the tax stamp is to Jack then Jill can't possess. But if the tax stamp assigns to Jack & Jill Trust, a legal entity with personhood, then Jack and Jill can both possess, because the Trust is actually in lawful possession. The trust also allows passing on the NFA item upon death in a sane way. If you own NFA items and don't have a trust, you did it wrong.

Mag Ban
Here's what OP is getting at; the mag ban will ban all transfers, including between immediate family members, which potentially causes problems. Is a dad tidying up the family gun safe an illegal "transfer of a high capacity magazine" if he pick up a magazine owned by his adult son? A plain reading of the current bill implies yes.

This is where OP's thought comes in: if all magazines are owned by a trust, then is it no longer a transfer when a magazine changes hands between trustees? That's the pickle.

Firearms are often owned by legal entities, and I'm sure there are private entities in Washington that own firearms allowing them to be transferred between authorized users. I'm 99% sure this is how many armed security companies work. They aren't going to a FFL every shift change, and there's no exemption, so it implies WA law doesn't consider this a "transfer".