I've updated the patch.
It now correctly places only the checked radio button in the tab order. If no radiobutton was checked initially, the first button in the group is placed in the tab order.
It is up to the developer to wrap the radio buttons in an element representing the group, with a meaningful name specified as either a "title", "aria-label", or "aria-labelledby" attribute. This can be any element (e.g. <span title="repeat options" />) as long as it's a direct part of the radio buttons, and a child of the element being turned into a buttonset. The buttonset widget will automatically add role="radiogroup" to these group elements in the buttonset. The patch also contains updates for two demo files (radio.html and toolbar.html) that demonstrate this markup.
In accordance with the DHTML styleguide, this patch enables the following keyboard support:
Only a single radio button in the group is part of the tab order at a time. The arrow keys can be used to change the selected radiobutton. If the Ctrl key is held down, the adjacent radio button is focused, but not selected. The space key can be used to select a focused radio button.
Note that this patch also fixes a problem where buttons built from checkbox elements were not keyboard accessible.