Fly-through video here. All spheres have a radius of 30 voxels.
I'm just using the abs(normal.z) as the pixel value and a very simple projection. This is a double version and there are some rounding/stepping errors present. I changed the maths slightly so that the stepping is parametric rather than on x/y/z which simplifies the code somewhat.
It's pretty slow on my laptop, about 1 frame/s, although it doesn't really matter how deep the volume is - this one is 4096x512x512. Not that it's optimised in any way mind you.
At some point I do intend to see how it would run on epiphany ... but right now i'm too lazy! The main bottleneck will be the tree traversal for each ray although I think in practice quite a lot of that can be cached which would be a big speed-up. Other than that it could be a good fit for the epiphany given how small the code is and that only a single data strucutre needs to be traversed.
TBH i'm not sure if it's ever going to be practical for anything because of the memory requirements even if it can be made to run fast enough. *shrug*