Apparently, you can mod the genesis 1 and 2 to work with s-video, as it is supposed to be the same encoder chip that is used on the neo geo, and genesis 3 has a chip that has s-video output, but that too also has to be modded (probably a lot less work than the 1st two genesis systems though). Gamesx.com has the tutorial for the genesis 3 mod I believe.
As far as just using a cable to support s-video, it can't be done without modification to the genesis.