^ Yeah it's true you can't fix that with the chromashift filter but you should be able to resize only the U (blue) plane i.e. like this:
(just a quick and dirty example, there're probably better ways to do it)
blue=last.bicubicresize(724,480)
mt_lutxy(last,blue,y=2,v=2,u=4)
you'll need the masktool filter.
Great find BTW with the chroma errors, they are indeed ugly and very 'unfilmlike' and really need to be fixed.