I plugged a USB wireless keypad into the front USB socket and it works,
it generates keypresses inside my xtra1-portal app (press RECORD in my xtra1 to see keycodes, from both the remote control and the keyboard)
I have tried again with a wired USB keyboard within your portal-xtra1, and it does indeed show keycodes.
Pressing F3 exits your portal and returns to the custom portal where the left and right arrow keys and Enter will allow you to select an option, however the Humax portal seems to have stopped working but this is probably not related, and generates a server not found message in the top left hand corner of the screen.
Pressing F12 whether in a portal or not, causes the box to crash and reboot.
So the keyboard seems to do nothing useful at the moment, perhaps things may change.
Edit: The problem with the Humax portal not working has been fixed by me turning on the HomePlug connected to my router in a different room. This should have been turned on by myself earlier using a remote controlled socket, which seems to work better when the remote control is pointing directly at it.