I like your thinking - that would be a nifty base kit for external video processing/colourisation! Since you have the VU003, you don’t need the Cadet III.
But you do want to make sure that the YUV video source is in sync (“genlocked”) with the input signal you use to clock the Cadet I sync generator/extractor. Otherwise the YUV source image won’t line up correctly with the output from the Cadet II encoders. I think you should be able to use the Y channel from the YUV video (which includes the necessary sync pulses) as the input to the Cadet I to achieve this. The Cadet I has a video out that is a copy of the video in, so I think this should work (sorry it’s a terrible picture but hopefully you get the idea):
