Alright ! I'm pretty sure that the problem resides in rkflashtool. I'll try to see if there's USB packets transiting with Wireshark.
I checked with Wireshark, packets are being exchanged with the device but it seems to enter some kind of infinite "SUBMIT <-> COMPLETE" loop with the device, sending no data during each 'SUBMIT'. I'll try to debug that this week, if I have some time.