You may not want to hear this, but I'd avoid planting anything of much size in this location. First, Japanese maples have rather delicate root systems and do not care for any disruption in their root zone. Second, the digging or cultivation in this area necessary to establish some larger, shrub-type plantings will inevitably damage some of the root system and this can rather easily lead to disease problems - it's an ideal entre' for something like Verticillium wilt. Finally, it is difficult to establish larger plants within the root system of an older, well established tree, maples in particular - water and nutrient demand is hard to keep up with. And the 'dense' shade under the canopy of the tree in leaf severely limits what would be happy there.

Go with some shallow-rooted groundcovers or offer to paint your neighbor's door. It will be far better than to be responsible for potential damage/possible death to this well established, beautiful tree.

Yikes. I didn't want to hear that.