I think the following system design.
USB Keyboard <---> USB Host shield <---> Arduino Leonardo <---> PC
USB Host shield - http://www.circuitsathome.com/products-page/arduino-shields/usb-host-shield-2-0-for-arduino-assembled
Arduino Leonardo - USB HID Keyboard emulation.
So "RasPi + Arduino Leo" is a kind of hardware keyboard remapper/tweaker.
Except usb keyboard data flow between Leonardo and PC, is it possible that Leo and PC can read/write another data flow?