Thanks to one of the readers of this blog, I got a tip.
He connected the MiniX via a serial to usb connector to his pc.
In that way, he can see the output of the bootloader and change the behavior of uboot via console. This goes a little over my head, but I hope somebody can develop uboot further so it can boot from USB. It is possible, now only unactivated he confirmed.
Here are the pictures:
As you can see on the right, the ground is connected to one of the pins of the USB port.
Credits go to Benjamin Larsson, thanks for sending this to me.