There is one clay that will not clog your pipes - it is pyrophyllite. Goes down the drain just fine. Some have even seen the calcium buildup in their drain pipe gradually disappear with frequent baths. Helps septics and city sewer as well since it encourages friendly bacteria growth.
The reason it works so well in the bath is because it is not a swelling, clumping clay like most bentonites. It falls apart in water very easily. I have watched 2 lbs of pyrophyllite go down the tub drain without a problem.