I know Kevin Horton managed to get something like that working a few years ago.  I don't think he ever posted a whole lot of technical details, though.

You can have a look at it here:

http://www.tripoint.org/kevtris/Projects/console/index.html

According to the page he is still missing a few memory mapper modes.  He has some documentation on the weird mappers used by a lot of pirate carts.

Otherwise, you're probably not going to find a better resource than the old stand-by http://nesdev.parodius.com/ .

Also, I'm not really holding my breath for it to actually happen, but there was a project announced a year or so ago to release an FPGA-based multi-system hand held called the GamePort.  I believe the plan was to simply reprogram the FPGA as you chose to operate as different systems.  It has now merged with some project called Wanga.

There's no real information at their site right now, though:

http://www.wanga.tv/

Good luck!

hey guys,
i have just joined and i like the concept.
sounds good to me!!!!!