I wish I could work out what I need to do to achive it just to impress the visitors
Simply dont add beryl-manager to the sessions start list.
Then when you want it, simply tyoe 'beryl-manager' into term, (or make an icon)
When beryl is running you can right click on the diamond in taskmonitor and choose which system will render the desktop. (gnome/compis/beryl) without having to restart the Xserver :)
I tend to run it by default then simply fast-switch to gnome desktop(using method above) on the rare occasions i need to have more GL power (ie when using blender etc)