I made a batch of EXP boards that plug into the bottom of a toaster NES.  One of the headers on the board is pin compatible with the Famicom EXP port.  You plug a 15-pin Famicom peripheral into a DB15M header cable which connects to the EXP board plugged in under the NES.

I'm still waiting on the 16-pin R/A header from Digikey, but here's what the board looks like...
http://www.chykn.com/ENIO-EXPBoard-v1_2-first.jpg

One problem - while I've verified that the signals are routed correctly, I don't have any FC peripherals.  Is there anyone out there with a Family Basic keyboard (or another 15-pin FC peripheral)  who could test this and maybe post a Youtube video for verification?

I've got a Famicom Basic setup if you need someone to test. I can also put a video up on my YouTube channel. Shoot me a PM.

super cool project keep us posted!

Thanks!  For those who are interested, here's the project page...

http://enio.chykn.com

Extended NES I/O.  The goal is to make an embeded ethernet/USB KB adapter for the NES.  The homemade EXP board pictured on the site is a bit different from the fabbed one in the link above.  I ended up moving the CPLD and 3.3v reg from the EXP board to the CPU board to make room for the FC EXP header.  Also added a 47K resistor between pins 3 & 9 on the EXP port for those who want audio in from the PowerPak without modding their NES.

Awesome project Pete. While I don't have a Fami Keyboard, I would be willing to help. If you need something, shoot me a PM.

Anyone who is interested, you can see the successful test video on my channel:

Thank you VERY much for testing it out and posting the results.  I've updated the project page with pictures of the latest versions of the EXP and CPU boards.  Got 5 CPU board PCBs in yesterday and built one last night.  The ethernet, USB KB and MicroSD read functions seem to work just fine.  Still have to roll the SD card bootloader and CPLD programming functions into the PIC firmware.

Shameless plug - if anyone is interested in an EXP board, shoot me a PM.  Trying to fund a batch run of the CPU boards! 

I posted the same info about this over at Famicom World also. I'd recommend posting your offer to sell the EXP boards over there also, as one person had mentioned interest. I'll be keeping an eye on this as I want to see how the CPU board and accessories pan out.

Have you tried video and audio out from the board? I'm curious if you can use a nice video encoder to get really clean video out from it. I notice that there are plenty of options with the way you designed it. Very cool.

Thanks again.  I put up a FS post here and put a link to it on your Famicom World post.

No, I haven't played around with the Video/Audio out signals on J5 yet.  Unfortunately I don't use a TV; my test NES is tied to a video capture card on my workstation.  I'm afraid I wouldn't be able to tell much difference if there were improvement in the video quality.

Yeah, I was hoping to give people a way to try out different mods without damaging their NES.  Originally it was only going to be for the ethernet adapter.  But then I figured I'd be a bit of a jerk if I was the only one with the EXP connectors and only used them to support my project.  This way people have access to all the signals and can do as they please.