@cedric-debuiselle
Sorry for the issue. However, we have found that the emmc is shipped with an older uboot that does not support overlays.
So the trick is to upgrade emmc uboot which is located on /dev/mmcblk0boot0.
A user kindly accepted to make some tests. We were able to flash (an apparent) newer uboot over the older one and it seemed to boot, but he forget to set the switch to emmc boot and booted on sd instead. He reported to mistake few days later...
However, he's ok to send me his emmc so that I can understand how to update the emmc uboot. So stay tuned!