You could score in a few inches around the outside to guide some wedges in. Why can't you quarter them with a saw, and then hew the saw marks off? Do you need the riving to show in the finished timber? Most of my hewn timbers are sawn first to save the jacket lumber. I have some 48' sticks coming in soon, and half the footage of those logs will be high quality side lumber, can't waste it hewing the timber.
