This was implemented in 3D Print Toolbox a long time ago, it's called Align XY located in Edit panel.
This should be integrated into the "3D print Toolbox" addon; this one: https://extensions.blender.org/add-ons/print3d-toolbox/
Great, but should be a native feature with knife tool and snapping enabled. To bad it needs an add-on (this one).
This should get integration of the "Lay Flat" addon; this one: https://extensions.blender.org/add-ons/object-lay-flat/