This feature is already available in the current spacedesk.
As of now, it will only work if Videowall is enabled.
On spacedesk Server window, please click the Settings button -> Video Wall Engine…
make sure to switch ON the Videowall then select the viewer connected on the left side tree view, then you will find a “Flipped Display (for mirroring)” check box on the right panel.
For assistance, please check Video Wall Configuration chapter in our user manual https://forum.spacedesk.netuser-manual/.