RT Systems are using the FTDI chip but have assigned a custom USB Vendor ID and Product ID to it. Under Linux you can use this cable (and I do use it with Chirp) by telling the ftdi_sio driver the additional VID/PID to look for. I have a UDEV rule that I believe handles this.
Is this a "version 4" cable? Someone came to me a few months ago with one that didn't even show up as a COM port on Windows when properly configured. It seemed like maybe it was being used in native USB mode to send/receive data to the radio and did not contain a typical serial port emulation device. This would make it harder for others to use the cable, of course, and would make it easier to lock the software to their hardware. Given that all their current stuff makes a big deal about only working with their cable, this seemed to fit.
I've never laid hands on one of these cables, so if I'm wrong, please correct me :)