A straight SNES-serial hack wouldn't work, since the SNES has 8 buttons + 4 directions & the GND line.  Only thing I can think of that'll work is a Genesis controller, since it has 4 + 4 & GND.  You may have better luck with a USB Gravis pad transplant in an SNES control.
Edit:  Also, that toyz4boyz site sells the EXT port connectors, so you may be able to bypass a breakout board.