Is it possible to get a camera to render in overdraw so I can get a cool map effect?
I totally know what you mean Envy! The Overdraw render thing looks amazing. Have you already found a way to render your scene like this!? I would love to do so too (but did not find a solution yet).
The shader for this overdraw effect is available here
Not completely sure what you mean by 'render in overdraw', but I'm guessing you want some sort of overlay effect?
If so, that can easily be done using multiple cameras and layers. By using layers and culling masks, you can specify what objects in the scene a camera will 'see' (render), and by using the camera 'depth' field you can control in what order the camera views are rendered.
For more info, search here and on the forums for 'minimap'.