Blender User Properties to PostProcess

I am trying to read user properties from blender into Unity, but I can't get it working.

 public class MyPostProcessor : AssetPostprocessor 
 {
     void OnPostprocessGameObjectWithUserProperties(GameObject root, string[] props, object[] values)
     {
         Debug.Log(root.name);
         for (int i = 0; i < props.Length; i++)
         {
             string propName = props[i];
             object value = values[i];

             Debug.Log("name: " + propName + ", value: " + value);
         }
     }
 }

I have already tried to export my models to fbx, dae, obj and 3ds. But none of those and not even .blend files call this method.

I have added my user properties in blender via the properties menu on the right (2.61) and added something in Object -> Custom Properties.

Is there anything that I'm doing wrong here? I would also love to use the collada format, but I have already found that there is no support for custom properties in Collada from Blender just yet.