Ok, I've got it working on a ATMEGA328P with Arduino bootloader. Thank you for your support !
I can read/write 511 bytes of data, that is enough for my use. BTW, what's the max size we can transfer in HID get/set report ? I've used 0x1ff (511) is it possible to go to 512 (0x200) ? As the max size in with USB_CFG_LONG_TRANSFERS=0 is 0xff (255), I assumed that 0x1ff is the max with USB_CFG_LONG_TRANSFERS=1.
Just need to add CAN ISO_TP code and it'll be ready. If you are interested, I will put the code on GitHub.
Thank you again.