AFAIK, libusb is a library that communicates with the higher level of the USB layer of the kernel. The lowest parts of the USB subsystem have to be written in kernel space, because they need to have access to the physical address space of the USB host controller and uses interrupts, and other low-level functions that are not possible to implement in user space.
So I don't think that it is possible to port low-level parts of the USB subsystem in user-space.