You could write a program in Processing (https://processing.org) to read the Serial Port coming from an Arduino. Say for instance, you press up on your arduino, it sends "up" via the serial port, a processing program register it, pressing a key.
This looks like it can do the trick. Now I can move on with the project.
The only thing that is annoying, is that it need a running/open software on the pc, to read the serial data.
(Not sure about this. I like it to work the same way as, some of my other usb device, which uses some drivers that is installed on the OS. Can it be done ? and if yes, is it hard ?)
vusb?
I used that and IR receiver to make HID keyboard out of old DVD remote.
this look like a really good tool to have/know. Can I use it with the usb-port that is build-in, in the Nano ?
(I have a shit load of old dvd remotes, so that could really be fun to reuse them for something)