Is there a solution for this yet? I currently use LifX Group of Groups smartapp, but that is going away of course.
I’ve tried creating a virtual light using @TAustin’s vEdge driver and then using Smart Lighting to mirror. The Smart Lighting app mirroring isn’t mirroring colors from the virtual light. It does successfully mirror on/off and dimming though.
Any suggestions?
There was a recent staff statement that, contrary to previous discussion, there will be a new architecture version of smart lighting and it will have mirror capability. So maybe this all gets resolved once the transition is complete. 
Meanwhile, as long as you have a hub, there is a community-created edge Driver specifically to enable mirroring. You might take a look at that and see if it fits your needs.
(New Release) Edge Driver Virtual Switches Mc (virtual Switch Mirror and virtual Switch Board)