I personally never knew there is a script to show the length of clip duration, but, what I usually did, I just double click* on the event, which will make selection of that event clip, and the duration length will appear in the lower-right.
*Double click on the center of the clip, NOT the edges as it will go to expanded mode editing.
Hope this can help as a temporary alternate solution while hoping the script or VP13 updated later to support this back.
The "Why" is because only View Extensions are allowed to have a user interface. If you create an Edit Extensions, it must do something without any user interaction (because vegas won't give you a window to display it in). Since most extensions interact with the end user, they are registered under View Extensions so that they get a docakble window.
Grazie, "Tools - Extensions" is the same: No display window. So Tools and Edit are no window while View is with window. You would need a function that requires no user interaction in Edit or Tools.
Auto Save in Excalibur is an example of such a tool. It silently runs in the background and simply saves every x minutes. I felt it was more of a "tool" option than an "edit" option so I put it under the Tools menu. While it does open a configuration screen so you can adjust the settings, it does not have an open window when used.