How can I map the Windows key to option in Microsoft Remote Desktop version 8? In Remote Desktop Connection version 2.x you could map the remote windows key to the local option key (instead of the command key) in preferences. In the new Microsoft Remote Desktop version 8 from the App Store I can no longer find this setting. Is this feature tucked away somewhere and I'm missing it?

One option might be KeyRemapForMacbook as discussed here, but I want to know if there is a solution that doesn't require third party software.
Indeed, this appears to no longer be possible without 3rd-party software.

Adding the same preference key to the new app seemed to not make any difference - it appears that this ability has been removed from the app.

As you mentioned, KeyRemap4Macbook would work with the private.xml that you linked to.