I spent the day running up three working Display servers for picture taking.
Everything seems to be working, except for a lot of tiny details that need to be ironed out: the samba share doesn't have the right permissions, the target login user doesn't have access to the X-server display, the Pi 4 build doesn't work on a Pi 3 and vice versa (different wlan driver, I suspect, and a simple apt-get upgrade might fix this).
I haven't run down any of the details because I wanted to get images for the project.
Three days left, and I think what I'll do is finish most of the documentation and images, work the bugs out of the system up to the HAD prize deadline, then take a few days to decompress and clean up around the house.
Here's the money shot for the Display servers.