Copper, chromium and arsenic are pretty poisenous to plants. They don't possess cytochrome enzymes and metallalothioneins that we do that help us detoxify poisenous stuff in our bodies.
Toxic metals compound sorb (temporarily stick to) soil particles, especially fine, silty organic particles like clay. Clay only looks red, because it has so much iron oxide stuck to it. If the soil in your box was really gritty, then the toxic componds in the pine would have nothing to sorb to, and would be flushed out each time you water. When they stick to soil particles near to a root, there is a chance that they can be absorbed by the tree of course.
I have a crazy idea! Put charcoal around the inside of the box. Any toxic metal compounds that leach from the pine will preferentially sorb to the charcoal and not the soil. As long as the trees roots don't infiltrate the charcoal, it should be ok.
But, if you're really concerned, plastic pond baskets are safer (but not as robust). But you know that already!
What do you think Alistair? I have a biological background really, not so much chemistry.