I would assume that the trainer cable will give standard digital signal
You can buy a Futaba Trainer Box that will work with Futaba FM,PCM and 2.4GHz "One universal Trainer box"
FM,PCM,2.4GHz is the TX>RX output/input signal "not the digital signal that is used internal in the radio "

You mention that the PPM signal is used trough USB that sounds right

Regards

la8pv