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 do not think, that the Mini USB host shield fits to the Pro Micro, but it is nothing what could not be solved with several short cables 
But with the keyboard encoder: why it is difficult to interface, what could be the problem here? To my mind, it is maybe easier to interface the keyboard encoder then to solve the USB problems by hand ... but I could be wrong 
A problem can be the number of pins, I need at least 30-35 pins, it can be solved maybe with a shift register ...
And another question, does it exists a good USB HOST shield for Arduino Micro directly?