Since MS-DOS doesn't support USB devices, I'd suspect that the BIOS would have to support the device natively as a bootable drive before you could achieve anything like this-- and even then it would ...

Im trying to flash the bios of a graphic card in my mac pro. The flashing part shouldnt be diffcult but damn, how the heck do I create a custom dos boot disk? <BR><BR>The mac pro has no floopy drive.