Thanks for your answer
First , regarding your remark of enormous file , i am of course limiting the size of the created image with the following
dd if=/dev/mmcblk0 bs=4M count=75 | pv > /mnt/openwrt/openwrt_$(date +"%Y%m%d%H%M%S").img
This way only the first 300mb are backed up (you can also gzip the file) which is enough (the boot fat partition is 20 mb and overlay is 256mb)
Also you get an exact full copy of your setup and you do not need to reinstall packages or what ever you changed.
As for my problem with "out of date files" in the created img file , I do believe i tried the sync command and it did not work. (i will try again i guess)
As for your second suggestion
mount -o remount,ro ...
you meant something like the following
mount -o remount /overlay
Can you supply a full command to force the overlay to be fully written to the sd card ?
Thanks in advance.