FFDSHOW uses (and contains) the libavcodec which is part of the FFMPEG project. I have no idea if this will include the splitter filters you need to be able to play back the NSV files.
From everything I can see, that appears to be a dying format (Matroska can do everything it could do, better, and unencumbered by problematic licenses). It also looks a bit like it is intentionally difficult to deal with (security through obscurity and all that).
If you have any NSV files, and you can't find a DirectShow filter that works, I'd probably recommend you work on converting them to a less troubled format (though that too might be difficult without the filters).