Reading the discussion linked in first post,that script is based on the assumption that avisynth's point resize can does "lossless" yv12->yv24->yv12 conversion only shifting the chroma, at least the point resize found in fmtc is the same.
From the documentation:
"point" Nearest neighbour interpolation. Same as Avisynth’s PointResize.
I admit that I don't know very well how to use zimg,
can I ask here how to upsampling 4:2:0 to 4:4:4 with the internal resizer?