Just a thought: once a user clicks on some radio button, it'll be selected, but the user will have no standard way to remove/undo this selection. The only possibility is to switch to another radio button on the same radio button group. As far as I know*, this behavior occurs on all popular GUIs, including web, windows forms, qt and gtk, and probably others.
It's been like that for so many years, isn't it time to add a "deselect radio button" standard? Just like a checkbox, a single click would be a simple enough way to undo a selection. For special cases, a "ForceSelection" property may indicate that this radio button group cannot be deselected, to preserve the current behavior.
* I merely researched this topic, it's a just-a-thought kind of post and might be inaccurate.
I'm not sure how much people will accept it, since than radio buttons will act more like checkboxes behavior, but you may wish to ask the HTML5 people to change the specification for radio buttons in HTML5 to reflect your needs. It should not be easy, but it is possible.
Isn't what you want just round checkboxes? Radio buttons are intended for A or B choices. Chicken or Beef. If you want to add "none of the above", that's a third radio button.