So, it does seem possible to boot the MiniX boxes from USB, the recovery mode has to be triggered. A10Linux reports:
A10 supports multiple device booting. After the system is powered on or reset, if RECOVER flag is set or button is pressed the processor will attempt to boot from USB0 port.
If RECOVER flag is not set or button is not pressed the device will attempt to boot normally and in the following sequence : SD Card, NAND Flash, and USB0 until boot success.
Now it’s time to do figure out how to set the recovery flag without flashing something.
More information here: http://a10linux.org/a10_intro.html