Hi WilliamG,
I did get Windows 10 working on an external Thunderbolt SSD on both of my iMacs eventually using LaCie Rugged Thunderbolt enclosures as well as ThunderBay 4 and Thunderbay Mini enclosures. The Windows 10 installer would not upgrade my external 8.1 installation, so I had to use WinClone to move it to the internal SSD and then move it back after the update and activation was complete. On the recent 1511 Windows 10 update, I had no problem with the external SSD on the 2012 iMac doing the update. However, I has the same problems with the 2014 5K iMac doing this massive update as I did with the initial Windows 10 install ... and eventually again moved it to the internal SSD to perform the update. As of now, I have not moved it back to the external Thunderbay hosted SSD yet as I am contemplating the risk of this again happening on the next update as opposed to just leaving it on the internal SSD. Small updates worked fine on the external SSDs, but the installer for the big ones seemed to have problems with just anything but a minimum standard configuration of drives.
I also have Windows 10 running in my 5,1 Mac Pro on a Velocity DUO PCI card SSD which is considered "external" since it is on the PCI bus. No problems with the 1511 Windows 10 update here either.