I've been working on this project off and on over the course of the past few months. Essentially what I've come up with is an Arduino sketch (originally utilized Uno, but currently utilizes Mega 2560) that "emulates" the SNES/SFC Multitap hardware.
There are currently timing issues with Player 4/5.
I will upload schematics (though its easy enough to figure out from the source) soon.
Arduino Mega 2560
The Arduino platform I'm currently using.
SNES Female Controller Connector
Controllers plug in here... (Sourced from raphnet-tech.com)
SNES Male Controller Connector (Need all 7 wires)
Plugs into the SNES... You may be able to mod an old controller to get this. (most controllers use only 5 wires)
Pullup Resistors - 10K Ohms
It works without this, but there may be problems when controllers are unplugged.
I've updated the source code on Codebender.cc in an attempt to speed up certain functions. I haven't hooked everything back up to the Arduino to test it yet because I'm using it for another project, and so I don't know if it'll help yet.
I'm sure there'll be some tweaking to be done once I do get back to it.