When trying to use the "firmware upgrade" funktion of the OEM firmware web UI, OpenWRT images are rejected. So far, we were only able to successfully flash using tftp from U-Boot, which requires a serial console.
The major difference between the stock firmware and currently built OpenWRT images is that the OEM firmware also contains a bootloader.
See in the wiki page for the device, mktplinkfw -i and binwalk show the differences between the OEM firmware and OpenWRT.
Goal is to create OpenWRT images that are accepted by the Web UI.