You can get root ssh on the device relatively easily by remounting the filesystem read-write and modifying /etc/ssh/sshd_config to point to your keys.

Placing something like the following in /autorun/95-enable-ssh.sh on the SD card should work (or in /autorun.sh on the PROXdongl3 USB drive), making sure to modify the echo “your keys” line. This script overwrites /etc/sshd_config, so if you care about keeping the old configuration for posterity/hacking purposes you should back it up separately.

#!/bin/sh
 
mount -o remount,rw /
 
echo 'Enabling SSH!'
echo "Port 22" > /etc/ssh/sshd_config
echo "PermitRootLogin yes" >> /etc/ssh/sshd_config
echo "PubkeyAuthentication yes" >> /etc/ssh/sshd_config
echo "AuthorizedKeysFile .ssh/authorized_keys" >> /etc/ssh/sshd_config
 
mkdir -p /root/.ssh
echo "your keys" > /root/.ssh/authorized_keys
chmod  0700 /root/.ssh
 
systemctl enable --now sshd.socket
 
mount -o remount,ro /