I don't think the leaves would work as a weed killer, as the toxin that prevents other broadleaf plants from growing under walnuts is spread though the roots. The effect of the nut washings on worms might be from the high level of tannic acid in the hulls. That might be what gets the tapeworms, too.
I have a big Manitoba Maple (you might call it a Box Elder) in the back yard that I've been meaning to tap for years. They apparently produce perfectly good maple syrup, just not in the quantities that you get from Sugar Maples. Too late now of course, but now that I know about the milk jug collectors, maybe next year.