There are several plugins that can mono-ize the master output. SWH's
Matrix Spatializer should do the trick. Or I think Invada has a nice
plugin.
Once you've got the plugin inserted and set-up, you can just
middle-mouse-click on the plugin to toggle it on and off. ( Kevin in the
video didn't mention this)
Ardour3 has a mono button for this purpose...
-Ben