UPDATE#2: Firmware v29 has made this impossible :( we gotta wait until Oculus releases an official passthrough solution for devs.

UPDATE: Firmware v28 has made this harder, but you can still access the hack. See THIS POST on reddit for brute-force methods of enabling AR (if it doesn't work for you using the method below).

TheMysticle has posted a video describing how to trick the Quest passthrough camera to get stuck in a transparent mode, so you can see both the outside/virtual world when running games. This is pretty ideal for the mapping setup, so I recommend for all users to check this out. It is very easy to turn on/off: CHECK OUT MY VIDEO SHOWING "AR" mode inside Custom Home Mapper setup.

TL:DR : When you peek through a normal guardian boundary, the quest will enter this 'AR' mode where you can see BOTH the passthrough camera feed AND the VR environment. If you disable the guardian (SETTINGS/DEVELOPER/GUARDIAN) exactly when this 'mixed-view' mode is active, it will stay stuck in this view and the passthrough camera will continue to render overtop of the virtual world.  I've revised the current mapping setup so wall and cube outlines will show up more clearly in this passthrough/AR mode.  This should be incredibly helpful for new users.