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.)

Edit: try this archived link instead.

Part 7 (auto-translated) (dead - archive here) 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.

Quietust's JavaScript simulation of the 2A03 now shows a 6502 sitting in the corner of the chip.

Attachment:

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)
(Edited to fix broken screenshot)

Interesting. Thanks for posting!
The auto-translation isn't bad at all (much better than some other stuff that I've tried to translate with google translate). The one word that was still in Hiragana (for some reason) is easy to translate: "donkey コング" is 'donkey kongu'.

-Tor