Live viewport preview of the active camera
In Unreal Engine, you can use a RenderTarget2D to capture the output of a camera in your scene. This can be used for minimaps, viewscreens, weapon sights etc.
I wanted to replicate that ability in Blender, and in doing so found the work of Kolupsy and Kaio on BlenderArtists.org, which allowed for not only live textures (RenderTarget2D), but also viewport previews of the selected camera!
The LiveTexture is saved as a data-block and can be assigned in the Shader Editor the same as any other texture.
The Viewport overlay will automatically start on clicking a Camera, and will stop on pressing Esc.
I will add a few preferences to allow finer control in the future.
You can also turn the overlay on and off manually via: View > PreCam or F3 > PreCam.