Not quite sure how I found this, but there's a posting here
which leads to a ten-part series in Japanese
about the genesis of the NES. (There's a promise of a professional translation.)Part 7 (auto-translated)
is particularly interesting: it covers the choice of the 6502 and the involvement of Ricoh. Although none of the engineers was familar with 6502 (and tools would need to be built), so the z80 was a much more obvious choice, the much smaller size of 6502 meant that the rest of the chip could be used for custom circuits. I think there's an idea that copying the design would set back the competition too, but the translation isn't clear enough.
now shows a 6502 sitting in the corner of the chip.
You can see the original die shots
on the visual6502 site, and the extracted layers and annotations on Quietust's site
(Edited to add screenshot)