Paul__B:
Sounds quite possible, however you don't need a keyboard encoder (which is going to be very difficult to interface) if instead you use a Pro Micro (Leonardo variant) which can implement a HID class in itself. This may still fit the "Mini" USB host shield (haven't looked at the connections).
I think i will try with this two: http://tinyurl.com/lh5eeyq and http://tinyurl.com/l3ky8uf 