I'm not sure I'd worry much :-) I grow ALL my JM's in containers and leave them out all winter and I have never had any issues (same climate zone - just a bit further north, in the Puget Sound area).

There are a few considerations you should be aware of......the smaller the container, the greater the chance of potential cold damage. And you need to be prepared for any extended cold snaps in which the temps stay below freezing for several days.

While the topgrowth of potted JM's can handle our winters with ease, roots are vulnerable to freezing temperatures. And roots of containerized maples do not have the benefit of the insulating factor of all that ground soil around them. That's why the larger the container, the more soil surrounding the root mass, the happier the tree.

Roots exposed to prolonged temperatures of less than 26F can suffer significant damage and can jeopardize the life of the tree. That's why you need to be prepared for any extended cold spell, so keep an ear to the weather reports :-) Grouping the trees tightly together can help - actually creates a mini microclimate - or wrapping the containers in burlap or bubble wrap if severely cold weather is expected.

Moist soil is slower to freeze in very cold weather than drier soil, so be sure potted plants are properly hydrated. Typically not a huge concern in the PNW in winter...........:-))

Make a pile of wood chips, bark or sand and plunge them into that for the winter. Then all you have to worry about is them rooting out of the drain holes, but if there is much of that likely you needed to put them in bigger pots anyway.

I moved to Portland (just south in West Linn) two years ago from NorCal and had the same worry - or should I say freak out. You should be fine. Come Dec I will corral all the pots and wooden boxes together in one spot (probably a cluster bomb of 15 or more conifers and maples on our deck) and leave them there til spring. When it snows (I am @ 500 ft.) I use moving blankets and wrap everything together. That is probably overkill though but I don't mind taking the precautions. Haven't lost anything... If it hasn't rained and there is a dry spell, and a freeze in the forecast, water everything prior to the freeze. I know that sounds crazy but the soil needs to be wet or else the roots could get damaged or at least that is what someone told me.