I made a very small fix to PreviewLayer, such that getName() returns the prefixed name for both Layer and LayerGroup, rather than just Layer.
Fix here: https://github.com/geoserver/geoserver/pull/976
It would also be very helpful if this could be backported to 2.7 (Although before today’s release seems to be pushing it). I am not quite sure of the procedure for this?
Torben