Most of viewers support PCHG (i tested mostra for example, ham lab can show and convert, visage, ppshow and more).
8 - 12 colors is valid for hires, for lowres limitations are significantly reduced - side to this HAM can be used, sprites etc - generally most advanced software should be able to do tricks similar to copperchunky/plasma effect - not sure if PCHG can be flexible enough...
IMHO counting colors on few lines is unavoidable as most of new color quantization algorithms trying to work on averaging method not independent pixels, side to this pattern errors detection/prevention is important (preventing dynahires stripes i.e. insufficient amount of colors).
Perhaps good approach is reduce size of picture to for example half lines and for example 8 - 40 transition of colors - this create crude low res color map that can be addressed by Copper/CLUT changes, side to this normal color quantization need to be apply but focused on details.