After sleeping on it, I'm almost certain that we made some mistaken assumptions. I think the micro-USB "data" port on the DFT-70 is really an OTG port, not an embedded pl2303 chip. This explains why your Linux box calls it /dev/ttyACM0 and instead of /dev/ttyUSB0. If this is the case, the pl2303 module is not needed at all, and it had probably been loaded when you had some other cable plugged in. To support USB OTG, your Linux box loads one of several modules whose name starts with g_  (g_serial, g_cdc, g_multi). I think the choice depends on the capabilities that the OTG device advertizes, or possibly one or more modules whose names start with cdc_  (cdc_ether, cdc_subset, cdc_eem).

you can test this theory by first removing the pl2303 module:

   sudo modprobe -r pl2303

and then plugging your FT-70 back in. If my theory is correct, /dev/ttyACM0 will come back into existence, but pl2303 will not reload.