So I decided to play with the headers I added to the board, and see if I could interface a terminal to the system. I used an Arduino Nano and several discrete chips to make a 4-port interface between the Z80 and a KB/display. Then I modified a Tiny BASIC hexfile to use the new ports, and voila! got BASIC running on the Z80 :)

I'll upload the circuit diagrams and Arduino code to the Files section of this project.

Here's a long video showing the details.