maybe, you might also be able to modify the current with "great stuff" expanding foam. lots of threads at cichlid forum about using that as well. again sealing it after it drys and you have carved it.
I've not seen people using the sculpty clay, so I'm not sure if you would have to seal it.
One thing to consider is anything foam will be very buoyant so you will need to weigh it down or attach it to the glass with silicone. Personally I hate the idea of attaching things to the tank...so for my things I'm planning on using rock or something to weigh it down