Well, think of it this way. Virtualization in the context we are discussing, is a hardware emulation. So Windows will "see" hardware as the Virtualization software sets it up. So, in theory, readyboost will cache Windows to run optimally on the "machine" it's running on by writing to the USB device for a disk cache.
Before inserting the USB device (thumb drive) start your VM, and once you are in then I would go ahead and add the thumb drive, or external hard drive. In the VM menu, make sure the Windows "environment" can see the USB port before you add the USB device.
All you are doing is letting the Virtual Machine know the USB device is available to Windows and Windows will not know any different.