I'd still like to know if it's possible to force a bus conflict to redirect the $4014 DMA to $2007 instead of $2004. Maybe it wouldn't work, but it sounds simple enough to be worth a shot.
A 2600 adapter would be cool. I used to have one of those for the Colecovision. I don't know if the NES has a video-input on the cartridge slot, but there were a few Gameboy adapters for the NES.
But an x86 CPU on an NES, yuck. Something that works like a 6502 (only faster) would be better. :-)
Color Dreams made a cart like this that used a Z80. They didn't finish any games for it because they figured few people would people would buy it, because it's high price.