If i remember well, it was working fine in previous versions. Very older ones, i didn't tested this recently.
If i try to drag and drop a null object or a camera as External transform to a Projection node in the Node editor, it doesn't work, i'm unable to drop it on the input field or even type the name in this input field. If i drop it in the same input field in the Attributes panel or material editor, it works.
With complex nodes structures, it could be very usefull to work only with Node editor.

Here a screen capture: