I wanted to share the news that during the ipywidgets sprint in Paris Jason Grout and me worked on an extension to (dynamically) load other extensions – no more rebuilding for developing JLab extensions! 
A simple refresh of the page will soon be enough!
The repository is here: https://github.com/wolfv/jupyterlab-dynext
And you can find some more information and our future ideas here: https://wolfv.github.io/posts/2020/01/19/ipywidgets-writeup.html
The future ideas include a snippet editor and extension manager to make it simple to start out with JLab extensions, and custom ipywidgets. We could also imagine adding a widget that can dynamically load + execute custom javascript in order to make the old Notebooks for custom widget development work again. Also, another mode for injecting new themes would be great (without the security implications of running custom JS).
Curious to hear what other people think!