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 ...

I've got a 4 Gigabyte flash drive that I'd like to make bootable. For right now, I need it to boot DOS or DS linux. I'd like to have it boot to grub or something similar so I can select either one.