How come electrical coupling always gets overlooked :[. Place two metallic plates on opposite sides of the wall. This forms a capacitor. The bigger the plates, the bigger the capacitor, and the lower the series impedance will be. There's an added bonus that the plates can be flush-mounted and can even be painted over on top.
Of course, this does depend on the frequency you're using, as you don't want it to be a resonant size -- as well as the thickness and material of the wall.