UPDATE: Aug 05 2016: Surprisingly, the Windows 10 Anniversary Update (Version 1607) installed on this installation okay, although only via the VM (running under VMWare Fusion). It would NOT install when running from the bare-metal installation (option boot from EFI partition). Trying to install from the EFI booted partition results in the 'Cannot upgrade due to unsupported disk layout for UEFI firmware' error. Once the update was complete on the VM guest, it also booted fine from the EFI partition. I have no idea how, or why this worked. ;-)
A success story is a nice way to start the New Year.
The internal drive on my MacBook Pro (Retina, 15-inch, Late 2013) has been getting a little full, and the Boot Camp partition was taking up about 45GB of space. I also wanted to run a proper Windows machine (including developer tools) and 45GB wasn't really enough to do much.
And so began my quest to see if I could achieve the following:
Install Windows 10 Pro (build 1511) on an external USB 3 SSD hard drive.
Have my MBP recognize this drive as a boot option when pressing the option key at boot (turning my MBP into a Windows computer).
Have VMWare Fusion recognize the external Windows installation as a valid Boot Camp source, allowing Fusion to run the same Windows installation as a virtual machine while in Mac OS X.