"This is my point of view:
These functions are easy to change: 
itemconfigure(), entryconfigure(), image_configure(), tag_configure() and window_configure()

It's only to add self._w in the proper place. Only one line per method"

At least Tix would have to be changed, too.

"Other third party extensions should not rely on _configure() because it's an internal method (it starts with underscore). We have rights to change the semantics of this internal method in any moment without notification."

But why insist on your rights, if there is no actual need to do this? The function I posted in my previous message for example serves the same purpose without having to change any other function call and without breaking any third-party code that possibly uses _configure(). 
You sure have the right to do this, but I feel it is at least somewhat unfriendly if it is done without necessity.

Besides, one thing I missed in my last post:
"Also it's more efficient to use this than the function _flaten() in that situation:
   
   if not self._w in cmd:
      cmd = (self._w,) + cmd
"
If you want to discard the use of _flatten() you would also have to change Misc.configure() .