I made a series of jpgs named nxNNNN.jpg, NNNN being 0000-1941 and a bmp with the backdrop
BASH command (should work in ksh also, needs imagemagick)
for x in *.jpg;do echo -n "${x}"\ ;convert -black-threshold 20000 -transparent black "${x}" png:-|composite -gravity center png:- c.bmp png:"`basename "${x}" .jpg`".png;done
This gives me a series of PNG files that I then load into avisynth with the oneliner
ImageReader("nx%04d.png",0,1941,23.976)
which I load into virtual dub under wine and then encode normally.
EDIT:
Sample clip (13 MB, XviD)