We don’t have very comprehensive docs for this right now. (Perhaps you could be the one to write a tutorial?)
Basically, you’ll need to write a JupyterLab extension that provides your custom cell class to the notebook. You can do this by providing an extension that overrides the default cell provider at https://github.com/jupyterlab/jupyterlab/blob/adf2f3f421906bfb61255fa74f783d33d6def7f6/packages/notebook-extension/src/index.ts#L277-L289. You’d probably inherit from the notebook panel content factory at https://github.com/jupyterlab/jupyterlab/blob/adf2f3f421906bfb61255fa74f783d33d6def7f6/packages/notebook/src/panel.ts#L294-L305 (which itself inherits from the notebook content factory at https://github.com/jupyterlab/jupyterlab/blob/adf2f3f421906bfb61255fa74f783d33d6def7f6/packages/notebook/src/widget.ts#L774) and override the createCodeCell method.
We’re happy to answer questions you may have.