I'm not exactly sure what you're asking. But Flatcar Container Linux is completely open source. In fact, everything we do at Kinvolk is. We do not build open-core products.
That sounds really interesting!
I currently work on a side-project which is a distributed application and I plan on incorporating kubernetes/cloud-native into some of the core concepts, so I will check it out once I'm at this part of the implementation!
For example, we've gone a step further than CoreOS did and have a fully open-sourced update server, Nebraska (https://github.com/kinvolk/nebraska). We also generate a list of contents and licenses for each build. Here is an example from the most recent stable: https://stable.release.flatcar-linux.net/amd64-usr/2512.2.1/...