Thank you all for your answers !
First of all we have found a workaround...
The USB stick has mounted on /hd-media/ and after iso-scan, the iso file on the /hd-media/ has mounted on /cdrom/ (loop).
But after network scanning, the /hd-media/ is unmounted... but /cdrom/ stay active and usable.
We need to keep /dh-media/ active to use postinstall script.
So the workaround is to comment the network auto configuration in preseed to let us configure it manually.
When the installation come to network part, we hit escape to go back to the install menu. And so we start the first step again (Scan hard drives for an installer ISO image).
The installer mount again /hd-media/ and /cdrom/ and come back to network configuration.
At this moment every thing is OK, we can continue to install and the postinstall script are executed.
This is the same behaviour that :
However, within the context of your chosen installation method, it is
sufficient to return to the third menu option (Scan hard drives for an
installer ISO). This will umount /cdrom, then mount /hd-media and
/cdrom again. You can then continue from where you left off.
The same result can be obtained with
mount -tvfat /dev/sdX /hd-media
mount /hd-media/<yourISO> /cdrom