I've been living with massive old black walnut for a while, and I've learned that I just have to try it and see when it comes to what can grow around it. I haven't tried buttonbush, but I've found that most
native species that I've tried that live within the same range as black walnut do okay - as long as they're shade tolerant, too. Like pawpaws, ferns, and others. That said, I've had plants that supposedly shouldn't be grown within umpteen metres of the tree's
drip line do just fine near and even under it. Tomatoes, peppers, rhododendrons, and honeysuckle have all thrived in proximity to it, while some supposedly tolerant plants (like jack-in-the-pulpit) have croaked.
My suggestion would be to plant one and observe it for the season.
I have told people my
experience and had them tell me that old black walnuts produce less juglone than young ones. I have done no research to back that up, but the logic behind that they give to me is that mature trees don't need it as a defence mechanism anymore, so they don't produce as much. I don't know if this is true, but it might explain why some people (like a cousin of mine) can't grow
anything near their black walnut while I've only had to worry about shade tolerance.