I posted the clay bath info to Slinky, down below. I only use one cup and do it in my regular tub. I have't had any drain problems. THey do say if you use 2 and a half or more cups you need to buy the special agent to dissolve the clay. I am so far using the minimum amount and haven't had to worry about that.
About it getting inside you, I am not worried about that. Although they say not to drink the clay, I don't think you would get enough that way to make a difference. It is mostly water, with some clay and herbs mixed in. And when you stand up, it will probably run out anyway. They don't mention any precautions, so I'm not worried.