I use a different technique now, but the bad news is it requires the purchase of some $40 software. My notes say thus:
It is now simple (but must still be a hybrid on the target drive)
Have more than 20GB free on the system drive, and use Boot Camp to install internally as normal. Install the Boot Camp drivers etc.
Now use
WinClone to shift it to the external drive (make sure it's GPT, partition as desired, use MSDOS format, then use WinClone to move the Bootcamp across)
That's it! Re-run Bootcamp to delete the internal partition.
If it doesn't boot and ends with a sad face, which seems to happen with Windows 8, after a few tries it'll offer safe mode. Try that. Or try winclone's “make MBR bootable” and try again including the safe mode.
There doesn't seem to be anything we can do about it needing to be hybrid and booting from the emulated BIOS. I have spent days trying. Newer Macs might, but mine will get stuck on the boot screen if a purely GPT/EFI based Thunderbolt is even plugged in. At least we no longer need an internal partition.