Hi Bear. Why I think that walnut is OK for smoking is simply because I can buy it at a local BBQ store packaged for smoking.
I did some searching, but all toxicity information I could find related to inhalation of the sawdust, or skin contact of dust, bark or leaves, not just for walnut, but various other woods that are commonly used for smoking.Oak, Maple, Birch. Beech, Alder all seem to be considered toxic to humans where inhalation the sawdust is concerned. How this applies to smoke I don't know as all I could find was the typical generic reply that wood smoke contains certain substances that can be harmful to human health.
As far as walnut is concerned, what toxicity juglone has to humans, whether from green wood, or seasoned wood, I can't say for sure. Perhaps, burning the wood does something to neutralize the juglone to the point where it has no effect on us. Yes/No?... I don't know.