Does the NES-CNROM board allow unmapping the CHR ROM from the PPU bus like the HVC-CNROM board reportedly does?
They are exactly identical, i.e. just a 74161. Unmapping the CHR ROM? Why would anyone do that? The '161 only uses D0 and 1 which we know simply select a CHR ROM page. If you wanted to, you could use another bit to control the CHR ROM.
I think the AC is referring to
this topic.