Very likely you could get away with at least 4 bits (16 distinct colors) per pixel, which is 4x more efficient than pure black and white.

I think the size of the effective chroma metapixels is more important than the range of values. You need to make them larger in order to keep the decoder from blending them together when upscaling the 4:2:0 chroma.

Now, if you're using a 4:4:4 format to do this, then you should be able to use smaller chroma metapixels (I still wouldn't use the full chroma resolution, though, unless you're using a high bitrate or a lossless codec). However, that risks data corruption if passed through a pipeline that downsamples the chroma.