S-Video should, theoretically, not require comb filtering, as the luma and chroma components are already supposed to be cleanly separated.
Are you sure the effect is dot crawl, rather than a regular interference pattern? If the pattern is very regular, do your S-Video cables have *both* S-Video and composite connectors at the other end? This can potentially cause a regular “netting”/checkerbox pattern to appear on the image due to crosstalk and coupling. I guess this sort of pattern can be eliminated with a comb filter, but it is not supposed to be there in the first place.
I definitely want to get to the bottom of this, as some Googling results in finding reports with similarities to yours pertaining to PAL N64 S-Video output over the last ten years or so.
(An excessively bright image, referred to in an older message in this thread, sounds like S-Video cables wired for the NTSC N64, but apparently cables proper for PAL N64s are now sorted.)