Booting android using 'fastboot boot'
This does not exactly answer the original question, but since I managed to brick my bootloader in the meantime, this is what I ended up doing, and it might be useful for someone else in the future:

Allwinner SOCs have a built in Boot ROM, and it supports various modes, so these devices are virtually unbrickable. In particular, it supports a so called FEL mode (http://linux-sunxi.org/FEL) which can be used to boot the device over USB. I nice open-source tool has been written that can communicate with the device in this special mode (http://linux-sunxi.org/Sunxi-tools#sunxi-fel). Using this, you can boot your own bootloader. (And with that, you can boot your own kernel, etc.) The linux-sunxi.org website has very  detailed instructions regarding Allwinner SoC devices, so I recommend that you check out their other articles as well.