If you look at the JS, the handle is preset to "se" and that is the only item that binds.
I've added the following to my ClientInit method which appears to lock it to vertical re-sizing.
All that remains is an IE Bug that causes the editor to shrink whenever you re-size it
el = editor.get_element();
width = (el.currentStyle || el.style).width;
// ensure proper width
el.style.width = width;
// lock resizing to just Y coords