Here's something that I bet you've already given substantial thought to - Given that one can't readily construct his own linear accelerator and that booking time on one isn't easy, it sure would be nice to be able to make Lichtenberg figures in acrylic without the use of one. At least in theory, do you think charge could be injected into the block by making the block be the dielectric in a parallel-plate capacitor? Granted, the capacitance of such a capacitor would be ridiculously small and the voltage one would have to apply to it to achieve the desired effect would be large perhaps to the point of infeasibility. Further, I have a feeling that the resulting figures, if you got any at all, would not be as visually appealing.