Sadly, it turns out I won’t be able to reach the initial goal which was to create a firmware for the DIY USB adapter. The reason for this is that the PS4 is not taking inputs from USB devices: it only takes inputs from the bluetooth side. This was confirmed by the developer of the USBXlater and the developer of the XIM.
Nonetheless, I should be able to make GIMX talk to the PS4 over bluetooth, using a cheap dongle. My plan is to make it work in Linux first as the bluetooth stack is much more opened on this platform (which means faster development). I’ll try to make it work on Windows in a second step. It should work in a virtual machine in the meantime.
Since the initial goal will not be reachable in the near future (Sony may allow USB controllers some day…), I am ready to make a refund. The early fundraisers have two weeks to ask for it.