canceling undo/redo events

I'm looking for providing a way for sites to cancel text modifications
caused by undo and redo.  It seems like the textInput event should fire when
the user invokes undo or redo.  Would it make sense to fire textInput for
undo/redo and maybe add a new InputMethodCode
(e.g., DOM_INPUT_METHOD_UNDO_REDO) for this type of input?  This would fire
before changes were made to the DOM and be cancelable.

Thanks,
Tony