I have never heard of clams doing this in captivity. When I was diving in Fiji in January I saw 2 small maximas, one about 1.5" and one about 2" and neither were sumberged in rock. All of the other maximas I have seen in the wild have been partially or completely dug in. I have no idea when they might begin this process, or even if every maxima goes through this process. I have read that they dissolve the calcium carbonate with some secretion at the bottom of their shell. I don't know much about this process, though I imagine it is very slow. If you were to create a hole and place a clam in there, I would guess that the clam would eventually grow to the size of the hole, and then you'd find stunted growth or a strange growth pattern, such as growing up but not out. I would very much doubt that they would dissolve the surrounding rock to accommodate their own growth. However, this is all a guess, and I really don't think anyone in the hobby can make a firm statement as to whether this is something viable or not. It would be interesting to try it and find out, but I personally wouldn't do it in fear of losing the clam. Good luck if you try it though, and let us know your results.