Yeah, the Luma/Chroma signals are for connecting to an S-Video jack. (AD724 datasheet shows the whole circuit.) Here’s a nifty tip… you could use the Luma signal as a rear RCA sync distribution output, in case you wanted to use Cadet 1 to drive LZX modules that require rear sync in over RCA.
For the squarewave out on the VCO, I think this is just the 5V TTL level signal. Just note that if you want to send it to a jack, you need a 3.92K/1K resistor divider and a video opamp buffer (as you see on outputs throughout all the Cadet schematics.) If not all that, at least an output resistor (499R-1K.)
You can use Cadet boards like a toolkit to build larger ideas. For example, you can get the square root of the triangle wave output from C9 by using a C10 multiplier to multiply the signal by itself! That creates an expo waveform (the basis for star wipes). If you subtract the expo waveform from the original signal at double the gain (using a C7 board) you get a parabola waveform, which is the basis for circle wipes. If you then use a crossfader (C6) to fade between the parabola and the expo, you get triangle (diamond wipes) again in the center of the crossfade. And that’s how you’d patch up half of the Shapechanger module’s gamma shaper. 