I don’t think a plugin can do that, and that is why it does not exist yet.

The UI part is not the issue, it’s more about what Sketch is able to do. I have a very short experience in plugin development, but i’m pretty sure that is not possible by using the core style functions of Sketch.

A plugin can easily add a border to a shape, but we can’t create part of “real borders”. Maybe, if you try to create lines or try to work around the inner shadow functions… In my opinion it will be a mess…