This is fantastic!
Have you found a way to determine whether the points are inside any arbitrary geometry? Or does it have to be defined by a mathematical equation, like the twisted cube? I was working on an identical “volumetric display” project to show models through particles, but haven’t been able to figure out how to calculate when the particle is inside the geometry 
. The best I’ve been able to do so far is to find the geometry faces via depthMap render.
Check it out: Volumetric Display