I wasn't aware of that development work taking place.

Reading the docs for the new "path" widget, it looks like the issue of different coordinate systems is resolved by removing functionality, as witnessed by the following:

"Canvases do not support scaling or rotation of the canvas coordinate system relative to the window coordinate system."

Which probably means that the canvas root will now be fixed and not allowed to have a -matrix option and be transformed, and therefore, bbox and coords will always refer to the coordinate system of the root. This is obviously possible with the present tkpath canvas too, if one restrains from using transformations on the root. But leaves canvas items without a native command to obtain coords and a bbox in their own coordinate system.

I might bring this issue up with the Tk developers at some point.

Thanks for the consideration.