Hi, although I've never tried what you are asking about, I thought about those little hand castings for kids. I still have the one I made in kindergarten.
Here's a couple of links I found that describes a few different methods. I would think either would be safe for your dog.
http://www.dltk-holidays.com/mom/plasterhandprints.htm
http://crafts.kaboose.com/dough-handprints.html
Good luck...if you do it, post a picture of it, I would like to see it.
Diane