I found a 1969 Rockwell patent [US3591836], "Field Effect Conditionally Switched Capacitor" which explains a lot about the fabrication of a capacitor, and also gives a nice detailed view of a contact.
Figure 1 shows that there are several thicknesses of insulator, and the capacitor is formed by using a much thinner layer than usual. The capacitor described is effectively unidirectional, but if the diffusion were extended all the way underneath the top metal plate, it would be an ordinary capacitor like the kinds we see on the chip. The depression formed in the metal by the very thin insulation section is visible in the image:
Next, I want to summarize my progress by showing the sections of the die I've mapped out and turned into schematics. The ROM and registers are obvious in the image, but since I haven't turned those into schematics, I'm not counting them as done.