@lisyarus @breakin FYI somehow the link in the last post was wrong (bad copy), corrected now. glTF is pretty easy to write once you know it, so don't really need a library but I found it handy to help.
Did you consider exporting to .vox? If so you can render in MV or my Avoyd, though these likely aren't as ground truth as Blender, they both do volumes well and should be unbiased.