That looks very interesting. Is there a rough ETA on a release?
However, looking at that screenshot a user will still have to choose his/her space and after that there will be authentication? For my use case the authentication would need to come first and only then show available spaces to avoid any data (even if its just a space name or dashboard name) to leak out as we want to give outsiders access to our data.
For now I might get away with using iframes. It will create a ugly double login situation (one for xpack, one for the initial landing page outside of elastic) and if you look at the page source you can still easily access kibana and show all available dashboard but if we use different indices at least they shouldn't be able to view any of the data. Downside is that managing all the different visualizations and dashboards for each customer will be a PITA.