Since mouse gestures came around in the 2010 beta I've loved them, there's just something inherently easy about pie menus where motion is an important element, it's less static and commands easily memorize and are fast to execute.
I think this is a good candidate for an "OK" / "ESC", in the middle of the menu, of course this would require a fair bit of research as to exactly how and when this would appear or grey out. Also the commands should likely be executed on mouse-release and not on mouse-over. A fast movement would execute a command, whereas a slow movement would execute an "OK" or "ESC" whenever it is available.
It's just an idea -- I made a quick mock-up in Photoshop. What does everyone feel about it?