The best part is, you probably wouldn't have to have much more than a 50mhz either...

. Much cheaper than a computer for MAME.
I thought about doing something like this for Atari 2600 emulation, except not putting it in a console case, but I still would have had hacked controllers for it(just like you're doing). I thought it would be pretty cool, but then I got a gutted cabinet for MAME and I haven't exactly had time for my other projects that I've thought of

There are people who sell pc's in NES cases though(or at least there used to be, not sure anymore), so it should be possible...
I don't know how to interface the controllers though, sorry

. I would think gamepad hack would be best, but if you want to keep the plugs, that could be a problem.