Recently I attended a gardening seminar and I learned that the walnut trees are not actually toxic. The Juglone is actually just the walnut trees way of keeping the competition down as it inhibits the growth of what is near it. So perhaps your Magnolia will just grow more slowly.

"...it inhibits the growth of what is near it..."

Except, like it the case of tomatoes (and others), it ihibits them to the point of virtually instant death. ;-)
Seems like symantics- if it kills something, its toxic to that plant.

tj

Juglone can inhibit some plants and not others. Growing up we had a group of walnuts with a lot of bulbs growing in the middle of them....it really depends on the plant.