Okay. Everyone made me do more work lol.
I found a pinout for the 7673
This seems to match the 7672 fairly closely, so it does appear that this is switch situation and not an EEPROM situation. It's an earlier set than I realized.
The main obstacle here is going to be whether or not a delay line is necessary. I couldn't really tell you that.
From the 7672:
So, if you need to add a delay line, the turn on voltage for the external video is ~3V. If you don't need a delay line, it's 5V.
This is on Pin 43 of the main IC. It was designed to just have a "video" switch on the front of the TV that toggled this. You might be able to find that circuit in the other TV you mentioned was similar.
However, I would still do the whole optocoupler thing if it was me. I don't want to drag out a heavy isolation transformer each time I want to use my TV, and I don't want to pay for one to dedicate to this purpose. Last time I checked, the cheapest Triad transformer I could buy for this purpose was about $65 before taxes and shipping from Mouser or Digikey. If you, jhalphen, have some inexpensive source for well made and reliable mains isolation transformers, I'd love to hear about it.
That said, it's still going to be a lot of work, and there's every chance that it's not going to play out as easily as you might hope it will. I would only do it if you are simply committed to the journey. If you just want a working TV, then you're time and money is better spent on a different set.