Thank you very much. Booting took quite some time, but in the end, see above.
For Future reference the commands:
opkg update
opkg install /tmp/kmod-mtd-rw_5.10.176+git-20160214-2_mips_24kc.ipk
insmod mtd-rw.ko i_want_a_brick=1
mtd -r write /mnt/sdb1/art_mdt8.bin art
Again, thanks to the GL.inet team for not giving up!
One final question: How Do I update the NOR part in the future? Via uboot? with the M16 Firmware?