> It can't have been hard for them to have looked through the graphics banks during the > approval process. It's not like it uses some bizarre one-off mapper or data format.
Generally for NES and SNES the way the approval process worked was that Nintendo of America or Europe played through the game for content violations and NCL (Nintendo Company, Limited) in Japan got freaky with the technical stuff. (Including but not limited to using bus analysers and oscilloscopes to make sure you aren't violating some timing margin somewhere).
In this case I can see NOA not finding the easter egg and NCL not caring.