The UARTs on this device are handled in Linux by usb-serial-simple. The simplest of the USB serial drivers on FreeBSD seems to be ugensa, and searching on the internet confirms that it's supposed to be generic (though sadly there's no description in the tree, even digging through git log for that file does not reveal anything interesting).
~~Unfortunately, at first I was stuck with the device resetting itself as soon as I opened the serial port. I've spent a whole evening wondering what the hell was wrong (looking at the bus in Wireshark and only finding a URB_COMPLETE Cancelled packet). Then I wrote a Python script using py36-libusb1 that simply reads the bulk endpoint… and I saw the expected console output.
Looking more carefully at what was "not simple" in ugensa revealed the "clear stall at first run" block, which turned out to be the culprit.~~
There was a bug in this device, but it was fixed in firmware 0.3.24.
maybe the device description should be set to something generic? currently it picks up the string for the first of the serial ports:
ugensa0: <Shell> on usbus0 ugensa0: Found 3 serial ports.
but "Shell" does not apply to the whole device, it applies to the first port cuaU0.0, while cuaU0.1 is "AP" and cuaU0.2 is "EC".