You could try installing Windows 7 RC1 to your internal SSD
, then boot into the Mac OS X Installer and open Terminal. There, you would type
dd if=/dev/disk0 of=/dev/disk2
This is assuming that disk2 is your external HDD. I would check that by right-clicking on the drive in Disk Utility and selecting Information, it should be listed next to the Drive Identifier.
As far as I know, the drive will be bootable at this point. There really is no concrete reason why Windows Vista/7 don't allow you to install to an external drive, since they are both capable out of the box to boot from those drives (unlike Windows XP, which required some modifications.) But once you get done with this process you could install Mac OS X again or, before you start this whole process, use Disk Utility to copy the Mac installation to a file and then restore it when you are done with this process.