Hi, I have already seen the same problem while developing sceneries and creating hard objects. It often happens when the shape of the hard object is complex. On way to deal with that is to split the object in 2 objects (with the same origin) and attribute the hard behaviour to the most simple shape. A rectangular or even just a flat surface will often to the job. If you use Blender, ctrl+J or P will let you split the selected faces into a separate object or join 2 objects. On export , you then just need to use the layers (select an object and press M to place it in a different layer). You can also not bother with the layers and show/hide the objects you want on export.
Also, the ATTR_hard_deck will let you taxi underneath the surface, whereas the ATTR_hard will project invisible walls below the surface.
Regards,
Guillaume