Ah, god. The problem was that unity-mir installed a dbus policy file that denied everyone but root the ability to send messages/signals to whomever owned com.canonical.Unity.Screen.
This is because it used "send_destination=com.canonical.Unity.Screen" instead of "send_interface=com.canonical.Unity.Screen", because presumably its intent was to stop anyone but root from using that interface.
Ah, god. The problem was that unity-mir installed a dbus policy file that denied everyone but root the ability to send messages/signals to whomever owned com.canonical. Unity.Screen.
This is because it used "send_destinati on=com. canonical. Unity.Screen" instead of "send_interface =com.canonical. Unity.Screen" , because presumably its intent was to stop anyone but root from using that interface.
Branch coming up.