Disable Z-traceability of an object in Cycles


I am using some volume shaders for lighting in my scene. I have one object/layer what is being lighted by an hdri environment texture, other objects are excluded from the hdri lighting with the help of a big sphere around the scene. I’ve done this by making two renderlayers and exclude one scene in one renderlayer. Combined together with z-combine node. However, i don’t want the cube to be affected by the z-depth of the cones. But is has to receive the lighting from the emitting cones. Regarding that last problem i read somewhere; Place the object in it’s own layer. Create a render layer, used only for the z-pass, and exclude this layer. For the other render layers keep this layer included. But i cant seem to get that done in the compositor. Can anyone help me with the right configuration for this. Please have a look at the file:

a1.blend (800 KB)

Thanks a lot!