There are the ways to do it for every platform and commercial apps that have features like Deskreen prove that. But they don't disclose their solutions. It is beyond of my current knowledge at the moment how they made it.

The goal of Deskreen is to bring more attention to this problem from open-source community, so then we can all come together and build a really cool stuff!

Of course I wanted to make Deskreen an HDMI Dummy plug FREE solution from very beginning but could not find out how.