The FIRMWARE upgrade is what allows Windows to run the Mac, not Boot Camp. Bootcamp is just a simple interface that guides you.
Once you install the firmware upgrade which allows for BIOS, then boot from the Windows CD (hold C during startup) and follow the regular Windows setup procedure while selecting your external drive partition to install Windows on to.
After installation, according to
http://docs.info.apple.com/article.html?artnum=75459 "Press Option-Command-Shift-Delete during startup to Bypass primary startup volume and seek a different startup volume (such as a CD or external disk)". At this point, you'll be able to select your external drive to boot from. Since Windows will be installed there, you'll boot Windows.
Note: I would use Boot Camp to create the driver CD which will allow Windows to support your Mac's hardware properly.