funky thing... not the way i wanted it to work...
i used plop 5.0.12-test1 iso for this test... just in case...
plop boots and prompts what I would like to do.

if i choose the hard drive partition 1 option, it boots up windows xp (of course)
if i choose the cdrom, of course, it reboots the cd..
if i choose the usb option, it attempts to search for the usb device and hangs at the uhci drive detection, as it find one device... [it passes ehci with nothing found]
do note, if i hold the shift down, it does the same as above, just skips to uhci then hangs.
now here's the wierd thing... if i choose the floppy, and there is no floppy on this system, the usb drive boots!
the only side effect is that now ms-dos has to re-think everything and everything needs to be reprogrammed as a:\ instead of c:\ ... on most comps, the system boots as c:\ [windows ntfs and linux systems] ... hope that helps some!