I managed to get the Mele A1000G Quad to boot from an SD-card, with a lot of help from Hans de Goede and Maxime Ripard.
The key to this is to press the ↺ (circular arrow) button with a pencil just as it boots. This will cause it to enter FEL mode, load the BOOT1 bootloader off the SD-card, and from there we have complete control. I’ve not yet got a fully bootable image, but it’s just a matter of getting some free time now.
Also this diagram of the FEL/boot process is very helpful.