If you want a cool looking 'scape, try it. Considering the weight you may want to consider the shape of the bottom side and if you need to take precautions reducing the pounds per square inch. Say it sets on three points, the PSI could be high. It that is the case, spread a layer of substrate, lay down a flat slate tile and set your piece on top of that, than cover the slate with more substrate.
On the filter topic, I would not use it with that purpose in mind. If it absorbs anything is unknown. It it does, in time it could become saturated like carbon does. If it does and once is saturated it could start releasing the over-concentration just like carbon does. The only true test is monitor your water perameters, before adding it to the tank, and on some routine after.