I just happened on your site. This is really amazing stuff.
In fact I'm sure I'll buy one when I get some money!
But I had a few ideas for experiments
1) Have you tried adding doping compounds to your acrylic to modify the current pathways? Seems like you could get a whole host of new patterns this way. Like maybe carbon powder or iron filings roughly stirred into the acrylic.
2) Would it be possible to generate the arcs in wet acrylic (before it is set). If you could do that it seems like it might be possible that the arcing process might instantaneously harden the plastic allowing you to extract the discharge pathways before the rest of the acrylic sets?
3) Have you tried it with acrylic that has europium powder added
4) What about placing high voltage coils on the sides of your blocks that would attract and control the direction of the discharge
5) Stacked Laminars might give you some odd results. Like thin films with partially conductive layers sandwiched between.
well it's all fun!