I'm not referring to the searching map. When you create a cache you get a map to select coordinates. This is the map I'm referring to, which does block out some areas due to hidden waypoints of caches.
Project-GC may not have any facility to provide the functionality you're asking for, because the Geocaching.com API is unlikely to provide hidden waypoints. Project-GC would have no way to find out these waypoints without the API providing them.