I think you may have under thought this. The ability to sight-read depends on SEVERAL aspects, starting from the easiest:
The ability to read, and play the notes quickly - first knowing the clefs separately, then being able to read both clefs at the same time.
The ability to understand the rhythm - first knowing the basic beats, then the subdivision of beats, then the ability to understand and play both rhythms in each hand.
The ability to keep your eyes on the music and to play without looking at the keyboard.
The ability to not just play without looking at the keyboard, but to also try and look ahead on the music.
They're the 4 basic principles I see in being an excellent sight-reader. To try and condense them into a DVD...
The problem is that some students have particular weaknesses and a DVD won't be able to address each particular student.