A I think that an 18" deep shelf supported by hidden shelf rods that are at the maximum 8" deep is asking for trouble. I wouldn't install a shelf any deeper than 12".
I would look for another way to support it entirely , or with traditional shelf brackets. If the shelf is within 3 ft. or so from the ceiling, you could always hang the two outside opposite corners with a small piece of 1/4" x 36" all-thread with a washer and nut under the shelf and then the opposite up in the ceiling with an appropriate 1 x4 block in the ceiling space; strong enough to support you proposed weight. then just three of your hidden brackets at the wall. No brackets showing , or maybe you have tile on the wall, and don't want to mount anything on the tile.
