I don't remember the thread but I do know that folks use large diameter PVC (large enough for the nem to feel comfortable and allow space to grow) and carpets -- I assume you mean haddoni -- have settled into them.
I am concerned though, about this statement: "and moves it around at will". I just want to make sure that people aren't thinking that these carpets are like a vase of flowers and that they can moved around the room (tank). Once a carpet settles in, it's because it has found a place that it likes -- that the water flow, lighting, and other environmental factors are to its liking -- and it shouldn't be moved.
I personally would house a haddoni in a tank with a deep sand bed, and allow the nem to move where it wants and settle into the sand.
If I wanted a rock dweling carpet (to facilitate moving the rock around), I'd consider a gigantea or mertensii, keeping in mind that gigantea are one of the hardest -- if not the hardest -- anemone to keep in a reef tank.
