Success! What I did was loaded an ancient OpenELEC with these instructions (and files):
With that, I was able to get on the network with Samba and SSH. Used SSH with credentials root/openelec to do:
tune2fs -L LIBREELEC_DISK /dev/data
Then came in through Samba and dropped the tar file for the new LibreElec release
in the Update folder, and rebooted.