The Lenonardo uses USB CDC, and can act as a peripheral/slave device on a USB channel. The USB Host Library 2.0 has the CDC(FTDI,Profilic,ect) drivers that should be able to send serial data on USB depending on the device its talking to(what chip it has).
I have another ongoing thread on a different subject. But I was curious if the drivers for the USB host shield using an UNO would be able to communicate with the Lenonardo? Or am I not fully understanding how the Leonardo works? I know it is using a microcontroller for its serial communication instead of a specific chip, but how would that effect things?