i dont think you can use usb multipass to boot windows setups, because they use their own bootloaders and require a ramdisk.
but then so do other live systems
anyway you can try multiple partitions, discussed in this topic (I thought it was impossible too)
http://www.911cd.net/forums//index.php?showtopic=18846
one partition with usb multipass and the other with windows setup bootloader
check out this for booting windows 7 from usb
http://www.911cd.net/forums//index.php?showtopic=18846
or you can try the BartPE way, but ur on ur own there is hardly any help on the topic
check out
http://www.911cd.net/forums//index.php?showtopic=23247
and
http://www.boot-land.net/forums/index.php?showtopic=8774
then use this for multiboot
http://www.911cd.net/forums//index.php?showtopic=18846
hope it helps