I had hoped that adding Composite Video to my little black and white Panasonic television would be a piece of cake, and in fact it looked like it would be a piece of cake. But it is not. It is not a piece of cake. It is not a piece of any kind of pastry.
After a lot of time playing around and poking and prodding and probing signals here and there, I have successfully gotten composite video to appear on the screen, sort of, by putting an ordinary 1V P-P composite video signal into the video driver transistor's base (pin 3 of IC12), and grounding the video coax to the input of the horizontal sync separator (pin 1 of IC12), which is just not at all how I expected it to work. But "work" is not really the right term, because it's obviously not really right; the video looks weird and very washed out, and no amount of futzing with controls gets it looking acceptable.
The problem here is twofold: One, I don't really grok analog TV circuits yet, and two, this TV is a hybrid between discrete logic and ICs. I think the circuit would be a lot simpler for me to understand if it were fully discrete, and I think it would be a lot easier to add composite input if it were either fully discrete or more fully IC based. But since it's a weird in-between thing, some of the functions are separated into ICs in such a way that I don't really "get" it. So, I think this will just be a TV I'm willing to junk so I can learn about how to drive a CRT in general, and not something for any specific project ideas.
That said, if you're curious here's the schematics and the IC details.