It’s definitely on the wish list! Unfortunately it’s not as easy as it might first seem. You can configure JupyterHub to launch jupyterlab c.Spawner.cmd = ['jupyter-labhub'] and pass additional arguments c.Spawner.args = ['--collaborative'] but the problem is how do you safely share a link?
In https://jupyterhub.example.org/user/user-1/lab/tree/foo.ipynb only user-1 can access anything under https://jupyterhub.example.org/user/user-1/, and this is enforced by JupyterHub, so we’d need a way to provide RTC access to one server without giving someone access to everyone’s servers.
Thinking out loud, maybe it’d be doable with a third-party hub extension/service that handles just RTC- and a JupyterLab plugin that communicates with this service?