Just had this problem.

I used a vice grip. Get covers for the grip itself so it doesn't mar it up. Put the stem in sideways, and use a large standard screwdriver to pry the ******* open. Once you get it on and realize that you pried it open too much, use the vicegrip to close it back up!

It helped me get a set of ergo road bars through a non faceplate quill stem.

I'm not sure how safe this is. Any input on that?