Bill
I tried once. I made a silicone mold...
I took 100% silicon and covered 1/2 of a very well cleaned stone I had. I oiled the stone and the caulk came off easily, but I didn't go high enough on the stone, and it made a very shallow planter. I planted sedums in it and they have been growing.
I filled the mold completely, and before it was set 100%, I carved out the inside. It was way too shallow! and I didn't put in any holes in the bottom for drainage. Felt the Tufa would be porous enough.
Try on a larger stone, and I bet you would have fantastic results!
Marcia

Thanks for the link, Bill. I'm inspired. If the weather ever warms up and the snow goes away, here's what I'm going to experiment with. I'll crumple up a 5- or 10-lb sugar sack lined with plastic wrap or a plastic grocery sack. I'll put a plastic covered cork or something appropriate for drainage in the bottom. I'll put the mix on the bottom and center a plastic covered pot over the bottom and add mix around the sides. When I've got the mix to the height I want, I'll further crumple the sack holding everything to create an uneven surface. When it's set, I'll peel off the paper and plastic and hope for the best.

I finally broke down and ordered the Prop makers mold handbook. We will figure this one out!! Thanks Bill