Initial experimentation with a Banana Pi shows feasibility, but it needs work. Booting a whole distribution and then serving the image is slow. I used a systemd unit on Arch to modprobe g_mass_storage with filename set to a symlink to an image, and to set the sysfs-controlled led show progress. It set it to heartbeat mode for 5 seconds when it was done and then to OTG activity mode after. Ideally, this will go on a Raspi Zero with just a minimal busybox rootfs and maybe a slim kernel so device boot speed is better. Then the HW UI will come.