It is nice to have you join us, Jen
I have seen relief work done in plaster, but the canvas they used was adhered to a firm surface to hold the weight. I have never worked in it myself, but I know that the plaster has to be reinforced for objects that extend more than 6" or so. Some people use metal, chicken wire and things like that. You have seen work done on freeways where they lay down metal rods for support. This would be the same premise.
Ah, make the most of what we yet may spend, before we too into the dust descend.... Omar Khayyam
Stone Chief Studio