Then I worked out what was wrong. When I was exporting the backplate and mask from DVD-Lab as AVI files, it defaults to 30fps as the framerate. I tried to export them again as 23.976, but it seems DVD-Lab only allows whole numbers. Odd. Anywho, I simply opened those two files in VirtualDub and changed the framerate. Then when I ran the avisynth script, well, everything worked.
Thanks again pittrek for the quick answers. I really appreciate it.
Now I hope that this thread can be of some use to others.