MacOS – Installing Windows 10 to an External Hard Drive SSD

I have a 2016 MacBook Pro, with Touch Bar, running macOS Sierra (10.12.2). I am trying to install Windows 10 to a new external SSD using Boot Camp Assistant.

I have read numerous methods detailing how to complete this installation, including this great thread.

Under macOS Sierra, this seems to be different. For example, when I run Boot Camp Assistant in macOS Sierra, the first screen is the standard introduction screen, and the second screen jumps right into the partition screen. I have no ability to download any software, or even to select my target drive.

This is as far as I have gone because I do not want to modify the partition of my internal drive.

Also, here is a screenshot of Disk Utility where you can see my external SSD currently partitioned and ready. I am wanting to install Windows 10 on the 100GB W10 partition (MS-DOS). The drive is plugged in, but Boot Camp Assistant does not recognize it (or allow me to select it as an install destination).

Any ideas?

Update 1:
I tried to get around this problem by trying to ignore Boot Camp Assistant altogether and just installing WindowsToGo on my SSD.
No luck. I eventually thought the problem might be because the SSD is split in 2 partitions, and maybe Mac doesn't like that. So I wiped the whole drive and installed WindowsToGo on the single partition using this guide. Everything appeared to install fine, but the MacBook will not recognize the drive when booting.

Update 2:
I thought I might be doing something wrong, so I tried using AAOMEI Partition Assistant Standard (which automates the WindowsToGo installation). Same problem. Everything on the drive looks fine, but the Mac won't recognize it as a bootable drive.