Given that you pour your own pan - lets not worry about the exact material but obviously it would be porous. Can you use a "waterproofing paint" to create your pan without any other waterproofing membrane?
This question Building on top of 3/4'' plywood subfloor touches on the topic. I have not personally done this but I am not sure I would be opposed to it. Given that Kerdi is selling at like 50 times cost to make and it is a long process to install correctly... Do we have the technology now to paint on our waterproofing? I am talking floor applications here.