|There is a more basic question that has to be answered. Does the OS you want to run have a boot strap loader program that will load the OS files from a USB device and does it have system drivers that allow it to run from a USB device? If yes, then it is only an issue of putting the boot strap loader onto a diskette and running it at the proper loacation in memory.|
If windows has such a loader, you could boot and run it off of a USB device this way.
However, doing anything with the USB interface like this requires a quite large device driver to access the USB interface. Not something normally done with a bootstrap loader.
If you build a boot diskette that can access the USB device, it could load the OS's USB bootstrap loader and run it.