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
(libreelec-amlogic_mx2.arm-9.0-devel-20190226160004-1bf54d9.tar)
in the Update folder, and rebooted.
Voila!