A short update: I figured out that Digital supports a fairly flexible Graphics integration called "Graphics RAM" that is a great way to test out CPU loops and timing, and also make demos more interesting. In the interest of keeping chip count small, I wanted to see if the TOM-1 could support just a single bidirectional transceiver (74LS245) for all of its I/O. I tested this out with Graphics RAM by first writing a byte for the address (into an 8x8 grid) and a second byte for the graphics value. With some additional circuitry, the TOM-1 can draw 256 color values onto a 8x8 Graphics display. The open question is whether it can do so using SPI to draw onto a command-based display, like Adafruit's SPI TFT display.