When selecting a center point for a new machining feature, it is not possible to reference geometry that is not visible. By activating the wire frame representation (by pressing the F6 key), this geometry can be made visible. Howerver, it is still not possible to project such geometry (PlanarCopilot=uv) onto the selected face for a new machining feature.
Workaround: by changing the setting PlanarCopilot to 3D it is possible to catch the center point.
It should be made possible to project geometry (setting PlanarCopilot=uv) that is only visible when the wire frame representation has been activated.