Thanks for the details rkomar.
The aim was not to install new os from a memory stick but to root the device and basically do the second method you described. Of course, it needs special skills and I was asking if someone has done it successfully before or at least studied the details of the question.
I was also asking if it is possible to root and hard format the hard drive before putting the new partitions, kernel and bootloader specific to onyx. Since Onyx firmware is open source, it is much easier to get the related information.