Enter Remmina. Remmina is the GUI front end for libfreerdp, the backend of xfreerdp. The problem here is that Remmina doesn't expose this feature, but as it uses the same backend (and we know it works) we just have to make this available to the user. This pull request fixed that in short order.
Note to self: Remember to check to see where programs are pulling plugins from when deving software. You could have had this PR submitted back in March.