Hey zhtet,
I believe our documentation on Depth expressions will help explain some of your questions in regards to Scene Depth. Keep in mind, only translucent materials can utilize scene depth.
Depth Expressions - Scene Depth
If you have further questions after taking a look at this documentation, let me know and I will do my best to provide you with an in ‘depth’ answer.
To answer your second question simply, yes. However, this does require a source build of the engine. What you can do is submit a pull request via GitHub. In order to get set up for this process follow the links and documentation below.
GitHub Setup
Getting Started: Engine Sources References
Let me know if you have further questions.
Cheers,