Maybe (and I strongly stress maybe) this will work:
http://ftp.us.debian.org/debian/dist...images/floppy/

Get the boot.img. That is actually the boot image for the Sarge installer for Debian and it loads USB drivers.. I won't have a chance to try it out myself until next week. There was also a DOS floppy somewhere on the web long ago but I had no luck with it. I'd be curious to know of any results you may have. I *think* there was a Knoppix remaster that had USB boot capabilities but that may have been for USB boot BIOS boxes only.