I have a walkthrough on how to boot Windows on a USB 3.0 drive without using Bootcamp on a Mac. It is here
michaeljjensen.com/tech.
It requires some setup with the partitions but isn't anything that would be hard to follow in my opinion.
I put this walkthrough together because this isn't documented anywhere that I could find. The only thing you need Bootcamp for is the driver disk it creates and you can use it to make your USB install for Windows. No need to worry about partitioning your main disk or the limitations of where you can install Windows now.