Live USB: Persistent Overlay full so system won't boot. Snapshot Invalid

[Rahul Nabar]

I accidentally managed to fill up the live-rw overlay and now my
system won't boot. It falls to the dracut emergency shell complaining
that it cannot mount the root partition.

Logs say:

EXT-4-fs (dm-0): unable to read superblock
SQUASHFS error: squashfs read data failed to read block 0x0
unable to read squashfs_super_block
Can't mount root file system

I've been trying to fix this following instructions at
http://fedoraproject.org/wiki/LiveOS_image#Merge_overlay_into_new_image
  The key step is:

# echo $'\x01' | dd of=/path/to/overlay-file bs=1 count=1 seek=4 conv=notrunc

But I'm stuck because I'm not sure what /path/to/overlay-file is for
me. Neither of the mentioned files seem present, at least in the
dracut emergency shell.

# dmsetup status does show "live-rw: 0 8388608 snapshot Invalid"

Is this the right way to fix this? Where do I find the right overlay
file to flip the Invalid bit at? Would love to get any advice on this.
Right now I'm stuck with a non-booting system and no way to recover
any data I had on it.