Always love to see a wagon be used for hauling lumber, show the truck guys that they dont need their pavement princess.
As for why there isnt a "real" solution to this, my thoughts are that its the same reason automakers dont put phone holders in their cars: it's a liability and giving any feature that could be dangerous can be seen as permission from the maker to use it unsafely.
Like, I've hauled tons of stuff in the back of my wagons before (my XC90 even had a split hatch with a mini tailgate, very cute), but it's not something I love to drive behind when other people do it, either. How many people do you think would safely use such a feature? All it would take is one not to and that's a lawsuit. Not worth the risk - especially when most of the hatchback car companies would be just as happy upselling you on their full size pickup models!
Sorry, turned into a bit of a rant
