I would think it would technically be legal in KCBS but using only the stems would make for a very odd looking presentation. As far as stems under the meat again would be legal, you might want to make sure there are still a few leaves left on it to identify it as parsley.
I've used parsley stems as a base in a box or two, when I ran out of lettuce and had a big pile of stems laying there, but they were just a base to hold the tops in place. I can't image ONLY using stems.. can't see how that would enhance the look of the meat.
Just a question bro, but why do you think only parsley stems would look attractive? It might be cool, who knows. Do a sample box and see what it looks like.