I do not know if your idea would work with a single coat of plaster and I don't know if there would be a problem with the weight of the plaster on the ceiling. I suspect that scraping the plaster off first would be a better idea if you want to use plaster stamps. However, you might be able to do this if you put on one coat to cover the plaster, let it dry and then do a second coat and stamp it. I'd suggest trying this in a closet or small bathroom area first. If you do it, I'd love for you to come back and tell me how it went in the comments! It is a very creative idea.