Hi Jedi,
I am afraid that is not possible in the current version. Changing the zoom factor and mode is not an operation that can be done to the image, it is merely a state of the window that displays it.
It currently takes two clicks and there is no hotkey for the editors, though there is a hotkey for the preview windows. Is there a special situation in which you would want to set zoom 100% that would benefit from an extra button?
V.