I don't think it's possible to run Windows from an external on a Mac. In order to run windows on Bootcamp there are drivers that need to be installed. What you can do is use WinClone to clone the Windows drive into an iso image then use bootcamp to transfer the iso over to that partition. But there is no way to physically use that drive as an external and boot to it from bootcamp. Now that i think about it, theres no way to boot into an external with Windows OS even from a Windows machine, well, at least not with a USB interface, not sure about FireWire.
Just a correction to your post Majesty,
It is possible to boot into an external drive/Flash drive from Windows. You need to ensure the BIOS allows you to boot from the device. If a computer won't allow it you may need to consider flashing your BIOS if possible.
I have a flashdrive that can run many verisions of Linux and Windows if I choose.