I don't think it's possible with the PC Engine or Genesis. With the PC Engine, definitely not. Both systems can do RGB, though; you need a mod for the PCE while the Genny just outputs RGB from the A/V socket (same pinout as the Neo-Geo, slightly different plug). If anything, you'd have to take the separate RGB signals and then combine them into S-Video, which would be counterproductive in the first place.
And yeah, I totally agree with the poster who said that S-Video is a relic. Once you've tried RGB there's really no reason at all to bother with S-Video. S-Video is a half-way attempt to clean up the garbage that is composite, and once you've tried RGB there's no going back.
- Matt