Thanks for the guidance agraham
I managed to suppress the HID in the Arduino as you proposed (I can see from the device info that it is not there and the Android keyboard opens - which it didn't before). SO some progress!
But still the Android doesn't receive any messages back from the Arduino. Pid and vid are as Leonardo 0x2341,0x8036, so I guess this line of code is still redundant:
usb.SetCustomDevice(usb.DRIVER_CDCACM, 0x2341,0x8036)
Then I tried changing the pid and vid in the Arduino to some random numbers e.g. 0x1111 and 0x1111
Now the Android doesn't recognize that a USB device is connected.
Still it works great with an Arduino with FTD.
Any idea what I could try next?