[Mpy]

There are two kinds of diffusion, turbulent and molecular. Both occurring in water. Molecular diffusion is almost just an idealistic concept, but is important in a few instances, primarily being water movement in interstitial water in sand and rocks, it is very slow movement but is important for nutrient transport through sediments(not being stirred by organisms).

It is completely wrong to say that water does not diffuse or move by advection. The best example to give is the Gulf Stream, it is the mass advection of a water mass through the ocean(like the return line from a sump), and along the side of the stream the gulf stream water diffuses into the waters surrounding it. If you did not have diffusion in an aquarium you would have no mixing or anything.

It may be that the engineering definition of advection and diffusion is not happening, but the scientific definitions are absolutely at play in an aquarium.

As to the question at hand, having a sponge coated in a thin layer of DIY dry rock, you would create a larger more porous center and the possibility for more bacteria to inhabit it. It wouldn't really pick up any more detritus than normal live or dry rock, it would just replace the center of the rock which would take years and years to have water seep into and become useful. Once you were able to get bacteria to inhabit the sponge it would likely act just like bacteria inhabited dry rock or ceramic media.