While I initially tried to get the Powershell Mount-VHD cmdlet working – I gave up as it didn’t seem to exist (despite obvious documentation: https://technet.microsoft.com/en-us/library/hh848551.aspx)

I was using a Server 2012 DS for testing purposes, but as I was just working on the drive letter side of things – I had yet to install the Hyper-V role. Turns out that Mount-VHD is exactly what I need to get the driveletter of a VHD and requires Hyper-V to be installed. The last example on the Microsoft Site gives you all you need to know:

With a little modification we can ensure that the System Reserved partition does not get in the way:

This is what you will get after mounting using the above command:

MountedVHD

You can then select the driveletter column to get what you need:

Much more simple than my previous attempt!

Enjoy.