In the attached scene you'll find one simple object called "backplateEnv" displayed as a purple box in the viewport, it is rigged to override the environment in the render with a background image.
This acts like a Maya Image plane linked to persp, but doesn't cast or receive scene lighting/shadowing (unlike the unusable image plane implementation in the Octane plugin).
To use it you can just import the scene in your scene and change the path of the image in the backplateImage node.
To disable it just hide the backplateEnv object.
To change the linked camera just change the name in the "camera" attribute of backplateEnv:
Technically this is just a sunsky node set to kind=visible + attribute "backplate" enabled + texture projected trough the camera rigged with a complex expression (as the "camera mapped texture" feature is still broken) + the "backplate enable" is linked to visibility of the transform.