Of course my fingers were itching too much and I was able to hurt my optfs at the point I needed a restore.
This time I tried to make an img file out of a rootfs tar with zlib compression as detailed some posts above. But, when I flashed it, the device resulted unbootable.
Too bad.
So I restored the tar as usual and lost 14 megabytes of free root space. Not so tragic, provided that we won't have a pr1.4 most likely and so I won't be bothered not to have enough space to install the update.

Any other tried flashing a zlib compressed root img? Success?