One thing to keep in mind. Whether something will or will not thrive under a walnut, black or any other type, is not the only thing to consider. The nuts and leaves will be falling on top of and getting intermingled with whatever is planted there. How difficult would it be to gather the fallen nuts and leaves in a Hosta bed? Same question for any other choice of plants. Maybe keeping the area semi-clear, either using mulch or a very low, ground hugging plant, may make life easier in the long run.
Just for fun, paint 2 dozen golf balls a nice earthy green color, go to a neighbors house, (with permission) lob the golf balls into the Hosta bed and then hunt for them.
