Sounds like an idea! With JupyterLab 3 it should be possible to have a pip-installable extension, and I’m hoping it’s possible to bundle the jupyter-notebook extension in the same package. It’s on my todo list to investigate…
If you’re publishing a JupyterHub extension what would be ideal from the mybinder.org point of view is if it could be customised if necessary, or perhaps extended i.e. use your extension as a library. I’ve no idea how to do that with JupyterLab though.