Sorry I got pulled away -- here's a link I found very useful:
https://www.anattatechnologies.com/q/diy-bitcoin-hardware-wallet-with-a-raspberry-pi-zero/
I know you can predefine a user, password, WiFi, and turn ssh on/off when writing the image with the Pi Image Writer (whatever it's called, I don't remember now).
Looks like that link is broken. Here's another one -- might be a mirror of the original, it looks the same -- https://tech.avant.net/q/diy-bitcoin-hardware-wallet-with-a-raspberry-pi-zero/
This one is also helpful:
https://www.circuitbasics.com/raspberry-pi-zero-ethernet-gadget/
So that would be running something else than Seedsigner...?
The guide suggested rasp lite, with gui stuff removed. Then electrum. I don't know how seed signer works, but with this setup it's mostly command line. You could set up a watch-only hot wallet to receive and monitor and create transactions. Export the unsigned transaction file, sign it with the pi zero, move it back to hot wallet and broadcast. It sounds involved but it's not that bad once you get the hang of it.
