Still trying to solve this problem for myself, this one seems most promising so far.
Take a top loading NES (the real one), have someone with skill (not me in this case) to A/V mod it and make the hole in the top big enough for a 72-60 pin converter. Done correctly, you'd have a a/v capable, NES that does both US and Japanese games.
The only thing better would be to find a way to make it do ALL regions, but I'd settle for that.
So, is it doable?