I haven't looked at the dithering info yet, but here's a version with raw out to try...
http://www.tateu.net/software/QTSource_RawOut.zip.
Avisynth must, of course, be outputing YUY2 and 2Vuy is the only setting currently supported (others, yuv2, etc., may be added later). You must have a codec installed that supports 2Vuy (Blackmagic) otherwise it will fail.
QTOutput("file.mov", format="2Vuy", raw="uyvy")
Raw out is running about 1.5 times as fast in my tests.