I completely forgot I'd posted this, which explains why it's so bad haha.
The new improved version of this script features a much easier to understand workflow that is more akin to how regular sprites work.
I'm not going to be posting any advanced documentation, but in short, everything works the same as sprites except instead of sprite.bitmap = Bitmap.new("path/to/bitmap") it's sprite3d.model= Bitmap3D.new("path/to/bitmap"). This system also allows you to specify the width and height of each frame if you aren't using squares.
There's also some weird other features, like "lighting" (lower layers are darker) which can be disabled, a pseudo perspective mode which can be altered slightly using another variable. If anyone cares about these I can go into further detail.
There's been a bit of fuss around mkxp in the fangame community (cough cough Marin cough cough Pokemon MK cough cough) and I personally love it. The performance is so much better than that of regular RGSS, and that got me thinking - would the voxel engine work? Well, yes and no. You see, mkxp has a limit on how large you can make bitmaps (which is 8192*8192, the opengl texture limit). This meant any spritesheets bigger than 8192 pixels wide would throw errors when trying to do anything.
This update changes the code ever so slightly so that the individual frames are transferred onto the bitmaps of each sprite, instead of being cropped with src_rect. This means you can create models with frame sizes up to 8192*8192 - in mkxp. While this update does slightly improve performance in standard RGSS, it's still not great when using large models.
Also note - while performance in mkxp is better, an 8192*8192 model would definitely be too much.