I wanted to restore a clean Raspbian image on the Raspberry Pi that was
running in the battery room. I could have gone and got it, but I would
have had to climb a ladder.
So I thought “Why not just remount the filesystems 'ro' and dd the image
to /dev/mmcblk0?”
I remounted / and /boot (after "systemctl isolate rescue.target", which
stopped sshd but left my ssh session running) read-only, and dd'ed the
image from an NFS mount onto /dev/mmcblk0. The dd worked fine. I used
/proc/sysrq-trigger afterwards to sync and reboot (I couldn't run any
binaries after the dd, which wasn't much of a surprise). The machine
fell off the network as expected…
…and never came back up. So I climbed up the ladder and brought the Pi
down to my desk and plugged it into my monitor. It did start to boot,
and got a fair distance before the kernel panicked. I didn't bother to
try to figure out the problem, just extracted and rewrote the SD card;
and all was well thereafter.
But I still think my plan ought to have worked.